Sorry, butis a kindergarden-level explanation/excuse. It is not 'just' unbalanced, but downright annoying, fluff raping, out-of-place perk ever.I could go on about the several useful effects it has starting ith the one mentiond above. But at the end of the day there;s really only one reason it needs: because.
You fight a 1hit hammer ASM as any other class (For this argument lets take a hbolter devastator). He lands in your face. He glance hits you, leaving you with like 20% hp. You somehow manage to kill the bastard (alone with ridiculous amount of skill and luck and/or with teammates helping you). The bastard dies right under your legs, denied the kill. He explodes and kills you, even though you did that hilarious fatmans' sidestep. Now, 2 things happen, one of it varies depending on the fickle mood of the Lag Gods.
1 - you got killed by lag. Now, it was awful during the first week, where evading things like that was useless as you'd get killed no matter how far it looked like (on your screen) you were from it. Exploding from ground kamikaze while you're 20 meters up in the sky as an ASM? Oh fun. Now, since more people have the game, it's better with the lag, but that doesn't nullify the problem. People with lag/bad hosts still appear, and if I am to play a game that's marked as 'competetive' where I win/die by a random chance, I want to pay a random amount of money for it.
2 - it's retarded. You managed to defeat a 1-2 hit kill melee monster that landed right in your face, only to explode and still get the kill. It's as if just running away from him was safer than disposing of him. You should not be rewarded for dying. Period. If you want to have a newbie perk, make it so that after 3-5 deaths in a row without any kills you somehow, raping the fluff so bad it yelps for Dark Eldar touch, instead.
Also, about the fluff rape. Now I know we have a few fluff purists on these forums, and while it's understandable that fluff =/= gameplay, it still should make some coherent sense. If in the lore Raptors/Assault Marines were to explode on death, there would be a terrible low rate of gene-seed recovery. And people don't rant at that? You might as well strap a jetpack to a rocket, save some time, training and resources. Or hire an Ork.
It's out of place.