All the perks have there uses in different situations.
Scrambler is pretty much a detriment to yourself if used alone but used as a team it can be very useful and frustrating for the opponents. Constantly having the radar scramble has them always second guessing where someone might be.
Hardline is good for a support guy on your team. 2 kills for a UAV ensures you will be picking out the enemy alot if they aren't using Cold Blooded. (It actually surprises me how few people use this perk. It makes the whole game alot easier imo.).
Last Stand is good for...well just being annoying

It doesn't work often but when it does its funny. I know myself how frustrating it is getting caught on a reload from one of these guys.
Sit-Rep is good for hardcore mode cos that is claymore city. It seems like every player uses scavenger and claymores in that mode. They seem to be everywhere, sometimes in the most random places.
In saying all that though I generally stick with Scavenger, Bling, Sleight of Hand, Cold Blooded, Stopping Power, Ninja over all my classes.