Before posting a response, I request that people at least read how it worked first. Please stick to the facts, and the topic, if you respond.

I am creating this thread for two purposes; to:

1. Explain the mechanics of how stealth speed worked pre-1.4 to people who may have misconceptions.
2. Request that devs re-instate the stacking of Nights Silence + Dark Stalker.

In the process, I will also examine the arguments I have seen for and against the stacking, as well as the pros and cons of allowing this in-game.

If you agree that it should be reinstated after reading the section on mechanics, click "Agree" or /sign the post, if you disagree, click "LOL" or /unsign the post. If you /unsign, it would be helpful if you state your reason why.

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The Mechanics

Some people seem to think that the stacking of Night's Silence +Dark Stalker could "make people run faster than horses sprint" and "enable them to escape from combat at will." Here is how it actually worked:

Out of Combat State:
1. Click crouch
2. Wait 3 seconds to hide if no enemies are nearby
3. The speed increase takes hold when the "eye" closes completely

In Combat State:
1. Run out of combat range OR use invisibility potions OR use Cloak
2. Stay hidden, do not take damage or get spotted (mages cloak and detect potions will break it), for approximately 10 seconds
3. If you manage that, you will drop out of combat state, the eye will close, and your movement speed will increase

As you can see, it was not a get out of jail free card. At best, it was a fun "get around faster when not in combat" card.

So what happened with the stacking, just how fast were people moving? Let's compare horses vs. speed:

(source in-game data &:http://forums.elderscrollsonline.com/discussion/129146/lets-clear-up-the-whole-stealth-speed-issue-w-vampires-hard-data-and-actual-testing-inside)

Base movement speed
No mount, walking = 100%
Sprinting = 125%

Horses
Standard horse, no sprint, no apples: +15% =115%
Standard horse, no sprint, maxed out apples: +65% =165%
Fast horse, no sprint, maxed out apples:+75% =175%
Fast horse, maxed out apples, sprint: +75%+25%=200%

Pros:
1. Horses can sprint indefinitely
2. You can wear any 5-peice armor set you want

Cons:
1. You can be seen at a distance
2. The fastest horse costs ~47k + 50 apples @250 each= ~60,000g

Stealth
Stealth speed, no tricks: 60%
Stealth speed + concealed weapon: 75%
Dark stalker + stealth: 100%
Night's Silence + stealth: 100%
Dark stalker + stealth + concealed weapon: 128%
Dark stalker + stealth + night's silence: 160%
Dark stalker + stealth + concealed weapon + night's silence: 186%

Pros:
1. You cannot be seen as long as you remain in stealth (unless someone gets you with mage light or detect potions)
2. You move 14% slower than the fastest horse sprints

Cons:
1. Moving in stealth eats stamina quickly
2. You cannot stealth in combat, it will take you at least 10 seconds of being out of combat before the speed kicks in.
3. You must be a vampire, which makes you take 50% more damage from fire (or give up a jewellery slot to lower that to 25%)
4. To get the fastest stealth, as a nightblade, you must use an active ability slot for concealed weapon, which is only good as an opener out of stealth, otherwise you are 26% slower than you otherwise would be
5. You must give up another 5-peice armor bonus for Night's Silence set.
6. You must either carry two sets of armor, or make Night's Silence your main set.
7. A white night's silence set will cost approximately 5000g from players, if you want a gold suit, assuming you are a risk taker that got lucky, and only used 3 upgrade materials each at purple and gold, the cost for a 5-peice suit at today's prices is:

700g for 1 purple upgrade material * 3: 2100 per peice
2500g for 1 gold upgrade material *3: 7500 per peice
Total: 9,600g per peice * 5 peices= 48,000 gold (conservatively), the reality is, if you want to be sure you'll get the suit on the first attempt, and count the yellow and blue mat costs, total cost is closer to.... wait for it.... ~60,000 g.

I can also confirm firsthand that the fastest mounts sprinting are faster than a nightblade running the full setup. I was grouped with two friends in Cyrodiil who had maxed fast mounts. I would gradually fall behind, then run out of stamina, then have to rest. One of them decided to start casting rapid manoeuvre on us. He would cast, mount, then sprint off ahead of me, when it ran out, he would dismount. Even running constantly, I was barely able to get into range to catch the rapid manoeuvre before he cast.

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Arguments for:
1. It's "fun" (yes, I put this first intentionally)
2. Anyone can do it (at least night's silence + dark stalker)
3. It is inconsistent that all other passives stack with armor bonuses, but this one does not
4. It appears "balanced" in that it requires 1) investment on a par with the fastest mount to make it viable in combat, 2) loss of flexibility by forcing the slotting of an active ability 3) increased susceptibility to fire 4) giving up an alternative 5-set bonus
5. Preventing the stacking makes the only "nightblade / vampire esque" armor suit obsolete
6. It fits within the genre (a vampire, that moves fast, wearing enchanted armor that makes you move fast, should move even faster)
7. It's a bit of fun that Nightblades enjoyed; this was huge for the poorest performing class at the moment which isn't much fun to play in it's current state (IMO)
8. It's fun.

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Arguments against (and counterarguments):
1. "It's not realistic" "It offends my sensibilities!"--> Please note, this is a game with vampires, not reality, if I wanted reality, I'd be at the office; in the fantasy genre, vampires are often portrayed as moving very quickly, as a vampire wearing enchanted armor that costs a fortune and makes you move faster, I should be able to move faster, it fits in the genre. You not liking it or not understanding it, is not a reason to change the way abilities stack, or for a non-combat setup to be nerfed.
2. "It allows users to escape combat whenever they want!" --> No, as you can see from the way the mechanics work, that is simply not true, it doesn't take hold in combat; the ability you're thinking of is called "Bolt Escape" and it is over that way --->
3. "But I paid so much for my mount!" --> People who use this set-up in combat sacrifice even more, and probably paid just as much.
4. "It's OP!!!!ROFLCOPTER" --> Why? In what way? All it does, again, is let you move faster out of combat, and there are significant trade-offs to being able to do so.
5. "The Devs may have some grand secret design we all don't know about that would make this OP" --> Given that it's not OP now, I don't really see how that could happen, but let's assume that's possible for a moment, how about we agree to cross that bridge if we come to it?

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Conclusion:
Night's silence + dark stalker lets you move fast stealthed, but not faster, nor as far, as similarly-priced horses are capable of, it also comes with some heavy trade-offs. But, it's fun. Please return it to the way it was in 1.3.