I think the commando complaints come from the fact that levelling commando is so much easier to obviously cockblock, accidentally or intentionally, than the other perks.
Support, berserker and demo all level from damage done, so it doesn't matter that much if someone else shoots at any particular zed you're shooting at. (Well, it matters a bit more for berserker, but most people are considerate enough to leave plenty of crawlers for them to deal with. -_-)
Firebugs suffer killsteals all the time, but if you're doing your job then things will be burning to death long before anyone else is able to reach them. And they also level by damage done.
Sharpshooters level with headshot kills against any target, and their weapons have the best range which gives them the option to target any head they can see, so having any particular zed taken out from under you is generally no more than a second's wasted time.
Medics will run into morons who'll heal themselves right in front of you, but these guys tend to be AWOL for most of the game anyway.
Between all the times you're defending against an attack, panicking about being surrounded and shooting anything that sounds threatening, blowing up a large group of mixed zeds, and aiming for something else entirely when one gets in your line of fire, the only reliable way to be able to avoid mowing down stalkers right in front of your team's commando is to be one yourself.