Think of the 5p TBS bonus as whatever bonus you'd get from an extra Mundus stone.
So, for example, if you normally run Thief, and you decide to run Thief+Apprentice, then you can think of TBS as:
2: add 1064 max health
3: add 967 max stam
4: add 967 max mag
5: add 213 spell damage (assuming 7p legendary divines)
If you look at it that way, it's not very appealing--you'd be numerically better off with Kagrenac+Thief or Julianos+Thief than you would with TBS+Thief+Apprentice. And this is why nobody should ever be using Apprentice if they use TBS.
One way to gauge the bang-for-the-buck of a particular Mundus stone choice is to compare its strength to a standard set bonus.
Lady/Lover: adds 1952 resistance; vs. 1935 resistance from a standard set bonus; Mundus stone advantage of +0.8%
Lord/Mage/Tower: adds 1952 health/mag/stam; vs. 1064 health and 967 mag/stam from a standard set bonus; Mundus stone advantage of +83%/+102%
Atronach/Serpent: adds 320 mag/stam recovery; vs. 129 from a standard set bonus; Mundus stone advantage of +148%
Apprentice/Warrior: adds 213 spell/weapon damage; vs. 129 from a standard set bonus; Mundus stone advantage of +65%
Thief: adds 16.8% crit; vs. 3.1% from a standard set bonus; Mundus stone advantage of +437%
As you can see, not all Mundus stones are created equal. Lady/Lover are basically junk. Atronach is pretty good. Thief (and Shadow, though Shadow has nothing to directly compare it to) are, however, in a league entirely of their own.
So, for example, if you had a choice between enchanting a piece of jewelry with recovery (169 recovery) and using the Apprentice stone (213 damage), or enchanting a piece of jewelry with spell damage (174 damage) and using the Atronach stone (320 recovery), you should do the latter.
There are really only 4 Mundus stones that are really worth using in this game, and that's Thief, Shadow, Atronach, and Ritual. All other Mundus stones are simply too weak.
The choice of using Thief+Shadow isn't just because higher DPS makes vMA easier (and safer), but also because the other Mundus stones just aren't very potent. You could consider using Thief+Atronach/Serpent if you have sustain issues. Otherwise, Thief+Shadow is the way to go.
Cheers! But I actually found for my own playstyle that the tower seems the best oddly enough. With undaunted mettle and boynd armaments it actually increased my stamina by 2033, providing a good balance between additional resources and more damage.
I just completed my first normal group pledge solo with it (yes, thats a milestone for me). So pretty chuffed