I'm probably reiterating a lot points that people have already said , but here's my perspective on this.
Kills CAN be an indicator of how well a player is doing in any FPS, but is never THE most important and relevant factor.
This varies greatly from class to class and map to map.
For example, if you are a commander (on a map with arty) and you very few kills (below 20 lets say) its likely a sign that you have not been dropping arty on time or not on the right spots.
Or perhaps you are an assault on Grain Elevator, sure you can have a whole bunch of cap points and team points, but if you're not killing a lot of people as that class on a close quarters map, you really aren't properly utilizing the role and frankly holding your team back significantly.
MGs are a little more complicated as often times the best spots are off on the flanks and out of the cap points, as such you generally won't have a whole bunch of team points, and since the amount of supression you are causing (which arguably doesn't actually "scare" people as some argue) is not really measurable, kills are the only way you can really tell how well you're doing.
Classes like Rifleman, SL, AT are the ones where amount of kills won't matter much at all, since Rifleman are the basic cap fodder and can do fine with or without kills - on some close quarters maps it can be a pain to rack up kills as rifleman
AT can only really realistically kill so many tanks in a 20~ minute round. SL's main role is to throw out smoke and pass on info to others and mark arty - all of which gets him no kills.