It would give me an incentive to actually do high level content instead of piddle around fishing.

Doing high level content means spending more time in game.

Spending more time in game increases the likelyhood of spending money on the game.

Spending money on the game keeps the lights turned on at ZoS for another week.

It's a win-win, really.

Plus, for the people who don't want to buy crown crates? Take old retired mounts and make them 1%/0.1% drops from hard mode bosses. You can either grind the content till you're blue in the face to get that cool mount you want, OR you could just go broke buying crates. The poor but dedicated players can feel special and accomplished, the big spenders can still get the exclusive 'crate themed' mounts, the mount remains rare as all get out, Zenimax doesn't have to make any new assets, everyone wins, everyone profits.

Yes, so long as the content isn't PvP only, or veteran only. If it's a reward for completing a public dungeon, then absolutely.

Its completely irrelevant to me, there are enough mounts in the game already with more being added every DLC or chapter, why do I need more? I already have what? a dozen or more? I cant even display more than 6 in my house. I much prefer the skins they offer as rewards instead. The only way I would be interested in mounts as rewards is if those mounts were actually faster, or could carry more, or had more stamina than the existing ones...and by a significant amount.

Getting a unique Horse for completeing the "Hero of the Daggerfall Covenant" achievement
Getting a basic senche mount for completing the "Hero of the Aldmeri Dominion" achievement
Getting a basic guar mount for completing the "Hero of the Ebonheart Pact" achievement

btw, the hero achievements require you to complete all the overland quests in an alliance.

Sure, mount, costumes, houses, etc

It would be a fun thing to earn a mount here and there, but not for just any "content completion" It would have to involve time/dedication so that it's truly earned, but not so difficult a casual player would never be able to complete it.

Because...More satisfaction ingame!! Plus having only horses from ingame means/stables is silly..
Ingame rewards satisfaction, goals, trophys, is a big part of the thing that keeps me playing a game ,and, really, the more satisfied with ingame rewards the easier my purse strings open for extra fluff = I dig, happy , want to support .
Zos should tip the scales a bit more in favor of ingame rewards anyway I think, cause too often I find myself grinding for things like furniture mats ( furn mats= besides decorative wax, an extremely rare find in biggest eu guild stores nowadays) or now the ashlander motif ( 0 out of over 20 quests) , and a tought enters my head " Zos So So Greedy... they do this to sell the stuff in the crownstore".
Now I understand a certain greed with business, but being too transparent with the greed can lead to the very opposite effect of the desired opening of purse strings, yes?

( Lotro and Rift for instance have very nice mounts rewards and I bet they get a ton of "returning" or on and of players when they have mount rewards from festivals. )

Oh yeah wanted to add one thing, I said ingame rewards such as hunting for goals and getting trophys are important to me= I forgot to mention that I like those to feel somehow tangible, well, visible to me , not like a title for others to see or the feel I completed an achievement for some abstract points.
Housing system is in the right direction with the house achievement requirements and furnishings, however those are pretty expensive to buy even after the achievement = I cant help but gets me back to thinking of the greed, hey buy some achievement trophies for 5- 25k or even 100k a piece , and then shop some nice smaller thingies to decorate your home with "for a few euros" from the crowns store after your ingame wallet dries out.
Can deal with this vibe being in game, but just saying it gives me this feeling, and that is not a good "marketing feel" to get a customer in the mood, is it?

More mounts, more the fun.