They don't give you the kill but instead give you a "Critical Assist" which is supposed to give you more XP for the kill so that was the trade off. Sometimes i will have a ratio of 4/1 for K/D but I will have 30 assists and Top Score.
Also, i think if you are being dominated and you get the Critical Assist you won't be dominated anymore but if they get just one kill on you after that they are right back to dominating you.
I think the Assault is a deceptive mech for people to use. Two vulcans is fun and can really put out the damage but if you are not fighting the opponent in the open you are at a big disadvantage. People think its easy cause you are just spraying all the damage but having no burst weapon makes this mech situational and really shines in certain maps. Anytime i fight an Assault now the moment i hear the the guns getting rev'd i just break line of sight and wait to here the guns stop. The spooling up the guns is a weakness that i see many people not exploiting enough and its one that i dont see most Assault players compensating for.