Fast Track rogue ability gets you Hasted for 2 rounds for 4AP.

So you spend 4AP, to get total of 6 AP recovery points.
You also do not get starting AP bonus, if you are already in combat.

Doesn't it seem like a waste of skill or I'm missing something?

You're forgetting the movement bonus. In addition, Glass Cannon doubles the recovery AP. However, the cooldown does seem a bit excessive.

So it's good for Glass Cannon builds? Ok, that makes sense.

To be fair, there aren't that many Scoundrel skills that are better than Fast track (with the exception of Walk in Shadows, Cloak and Dagger or any CC abilities). For example: Daggers Drawn, Lacerate, and Venomous Strike. Daggers Drawn is terrible because it scales with nothing. Daggers Drawn doesn't count as backstabbing even if your positioning is perfect and it isn't modified by Guerrilla if you're sneaking (you could do more damage with 2 backstabs for just 4 AP than you would do with Daggers Drawn for 8 AP, which is ridiculous). Lacerate and Venomous Strike deal pathetic damage relative to their cost. You'd be better off spending the AP on just sneaking then backstabbing (or CC to keep your target locked down).

Anyway, as far as I've tried, the best way to get most from Fast Track with regular non-GC build is to cast it right before battle and get to fight in same round.

That way, you get 3+4 = 7extra AP total.

While if this is done in combat, all you get is just (2*3)-4 = 2AP.

I mostly just use it when it would be impractical to attack (e.g. if my group is pulling a large group of baddies towards me). It seems to rarely make sense to invest in the long term when you could be removing nasties from the fight immediately with those AP ^^