1) Not too many people use heartbeat
2) If I see you and you see me, and we're close together, I'm going to knife you. Not to mention getting out of a jam with commando, too, you know when you run through somebody's bullets and stab them.
I use ninja, too, but only on maps that you usually won't be too close to a person (like Wasteland).
If you have cold-blooded and ninja pro and sneaking up is your tactic that works, that's fine. But other people use other tactics and work just as well if not better (depending on the person).