I don't think they're thought out very well. Firstly, has anyone ever reached lvl 4 anyway? Mostly I'm stuck on 1 sometimes rank up to 2, maybe 3 once or twice, don't think I've ever gotten to 4. This makes Anti-Tank for Engineer all but useless as you get extra mines to carry at lvl 1 but can't lay any extra mines until lvl 3 so you're just carrying extra mines you can't use until your other ones are destroyed. I try to run with class specific field upgrades but I bet 9/10 people just run with defensive or offensive because the rest are pretty useless unless you somehow get yourselved levelled up... which I don't even know how to do anyway, I assume it's just killing while not getting killed?