A little late on the third perk discussion (I was in class), but I make use of 4 of them (not last stand).
Commando is probably my favorite, because it suits my overly aggressive playstyle. I love playing runner classes, as well as rushing classes (note, two very different things), and it can also come in handy for general assault classes. On some maps, its next to useless, such as Wasteland. But in places like Favela, where you can be shot from 8 different directions, jumping off the building into an alley without getting hurt can mean the difference between life and death. Also, it is essential for knife runner builds, between the extra lunge range and the batman style shit you can pull off, and it helps for when you try to do parkour style jumps (again, a Favela trademark) and fuck them up. The extra knife range for assault style players is also important, especially if you are not using a silenced weapon. Sure, I could use my gun at ten feet and get the kill, but then his teammates are more likely to discover me. Sometimes it's better to go for the knife kill, even if you are pushing your luck with the distance.
Ninja is a godsend for stealth builds and or defensive/camping playstyles. I always have ninja on my Nuke-Hunter build (as well as a silenced weapon) to protect against those pesky HBS when I'm busy using my chopper gunner, and if you want to go ghost, there is no other option. The silent footsteps and HBS immunity makes you practically invisible to the avergae player, especially with cold blooded in combination. Its an excellent alternative for runner builds, but I prefer commando due to the unbelievable verticality of many levels. In more flat areas, like Wasteland or Sub Base (where most of the drops wont kill you) I find ninja to be an excellent choice.
Sitrep is far more conditional than the other two. It really shines in hardcore. Keep in mind that if a claymore is planted outside, and you are outside basically anywhere on the map, you will see it. Sitrep "awareness lines", from what I can tell, seem to be dependent on doorways. So, if you are in a building, you can generally only see other claymores in the building or that your building has a sightline to. But the minute you step out that doorway, it will light up from across the map (eg in Quarry, I have exited the large red warehouse by the large door and been able to see claymores by the blue building with the forklift at the window by the sniper ledge, or those over in the mobile SAM shed). I dont know if thats exactly how it works, but it seems to be something along those lines. In hardcore, where everybody and their mother sets a claymore on each door of the building their camping in, this is basically free radar at all times. The added bonus of louder footsteps is just icing on the cake. This is my default green perk when playing HCTDM because it provides so much info about the enemies location. Regular, things tend to be much more fast paced with less camping/"strategic positioning" and it loses a bit of its effectiveness, but the footstep side of the perk is still excellent (though it doesnt cancel out ninja from what I can tell).
Finally, steady aim is probably my least used of the four "useful" (imo) green perks. It's pretty much exclusively dedicated to my sniper class, or my Runner will sometimes use it if I've decided to switch the knife for a SPAS or G18s. However, keep in mind that it also increases mobility in a firefight, since you are not forced to ADS to hit your target when using an SMG or similarly sized reticles. Like Rubx said, generally you start shooting and then pull up into sights to finish the kill. But if you have one enemy right up close and another 40 feet away who both have eyes on you, you sometimes need to get just that first kill and then duck into a doorway before number 2 takes you out. This is where steady aim shines, and it is far less useful than the other perks just because of how seldomly that sort of situation arises, at least for me. Usually if theres two enemies, I've noticed them both and am prepared for it. Its just when surprises hit that you need to be a little more mobile (or a little more lucky, whatever works).
Last stand...well, I hated it in 4 and still hate it now. If the enemy is smart, he'll leave you to bleed out in ten seconds rather than chase the kill around a corner where he knows you're waiting. Sure, I've died a few times to guys in last stand, and it drives me crazy in hardcore when I drop a guy and dont expect him to shoot back, but generally speaking I just dont find it that useful for me and my playstyle.
Finally (and just for you Rubx) though you love ninja because you use headphones and can hear the footsteps, keep in mind that the vasy majority of players dont take advantage or dont pay attention to the excellent sound design in the game. Many times when hot-swapping with friends, who still have surround systems where I can clearly hear the footsteps, they will pay no attention. Very few players, when they have a UAV in the air, will crouch down to minimize noise when approaching an enemy position, as they think its unimportant. I agree with you completely that the sound of footsteps is a dead giveaway to the players who pay attention to it, but unless you run up against a clan or a really attentive player, you generally dont have to worry. I consider ninja to be the second most essential green perk in HC b/c of HBS overload, but in regular I am far better off with Commando or steady aim to help me kill things than Ninja to help me avoid being killed.
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