It's not unique to that class.
In the core rules, it is. I can't speak to what classes outside of the core rules have it, but I'll bet money they're monkish in flavor.
Also, that's NOT what IUD is for. IUD is only flanking. Not grappling, not climbing, not anything more than flanking.
All right, got me there (and I was looking at the book; shame on me). I'm not entirely sure that UD should apply while grappling, but whatever.
1) It's not merely the fact you're giving away key class features that hurts the rogue. You're also making defenses against his only real method of killing things much easier to obtain, when it's already a long list of opponents and items that grant you sneak attack immunity. If a Rogue can't catch you flatfooted, it's much harder for him to fight you, astronomically harder if he's not a melee rogue. I've played a lot of Rogues, aside from enemies with natural immunities, the toughest foes for me to contribute in combat against are other rogues and similar. It's just so hard to sneak attack them!
Well... if you're going to change things around, why not limit/eliminate immunity to sneak attack?
2) I'm really sick of the whole "Fighters are the weakest class," "Fighters should be good at fighting!" (fighting ambiguously used to reference any little possible incidence that could happen in a D&D fight), and other such arguments.
They are (possibly second to the sorcerer), and they should be.
Rogues are not much better than FIghters at higher levels, and absolutely weaker at earlier levels.
In a straight-up fight, yes. Rogues, however, aren't meant to wade into battle like fighters. That's why they have evasion and high Reflex.
UD isn't much use to an "optimal" Fighter anyway, at least for AC.
What's an "optimal" fighter? One who wears heavy armor?
As a form of immunity to sneak attacks, a form of attack that if you follow the suggested population distributions of PC classes the DMG suggests represents a miniscule number of potential attacks...*sigh*
See my suggestion above about sneak attacks.
If you want a more agile, reflexive, light armored fighter...play something else. Like a Rogue.

Or a Swashbuckler.
Why? What if I want to play a more agile, reflexive, light armored fighter, who does nothing but fight - not picking locks, opening chests, searching for traps, and such? I don't know what the Swashbuckler does, so this might well cover it, but what if I don't own that book?
What bugs me is the assumption that every player/group has access to any or all non-core materials. The last group I was in was a bunch of folks who'd never met each other before; the DM decided to limit it to core rules only until we got used to each others' playing styles. Unfortunately, the DM became unable to attend after a few months and the game stopped, but my point remains - not everyone uses non-core rules.