As pointed out earlier in this thread:
1) Bad for leveling (can't backstab a mob unless it's stunned)
2) Bad for soloing old content (can't stun bosses so you're stuck spamming Hemo)
3) Can't (usually) Backstab totems (Shamans in PvP - especially Resto - take a glyph to give all their totems 5% of their life as hp, and some totems like Fire Elemental Totem have over 100k life).
4) Can't Backstab Void Tendrils, so your dps is too low to actually break them before the root timer ends
5) There are some mechanics in raiding that require you to stack in a certain spot often in front of a mob, so you can't Backstab it
6) When boss mobs spin to target another player, you can't Backstab them
7) You can't Backstab pets in PvP if they are focusing you unless you stun them (waste of CDs, not to mention they can dodge/parry/block your Kidney).
8) Some encounters in Brawler's Guild are virtually impossible for Subt because the mob always is on you.
9) Even if a mob is stunnable, after a couple stuns, there's 20 seconds of down time where you can't Backstab until the DR resets.
10) How do you get "behind" a Warrior while they are Bladestorming?
11) Spell effects and pets (like Fire Elemental) can completely engulf a mob to the point that it's literally impossible to tell which way they are facing
12) There's a number of mobs in the game that have no obvious front or back, so the only way to find it is through trial and error
13) In PvP, many classes have slows equal to or even stronger than our 50% from Crippling. Mouse turning is instantaneous, while movement is not, meaning a good player can make it virtually impossible to get behind them to Backstab them.
14) Ever see a player lag and keep moving or spinning while stunned? Try backstabbing them.
15) Lots more I didn't think to mention