These are pretty common pet peeves, and its easy to understand. Let me try and add my 2 cents to them for discussions sake. I don't necessarily disagree on any point however.
The medic class gets 2XP per heal, unless the game decides the target of the heal was in dire need of it in which case you get a greater number (I can't remember the number, its quite decent however - like 15?). On the flipside, you get 8XP for killing a clot on Normal difficulty. On earlier waves of the game 1-3, you should be generally capable of keeping yourself in check. If not, your probably being a bit too reckless and should reevaluate your playstyle. Expecting a medic to heal during these waves and make that their priority IS a reasonable expectation. But the game puts so much emphasis on kills over healing that it's also reasonable for medics to expect you to look after yourself (generally - I'm really only talking about Normal/Hard here) so that they too can get some XP.
Some perks can really shine when played well, off on their own. I've seen many a Commando go and stand around the bend from the group and absolutely nail it - however this requires a lot of communication to understand that this is an intentional strategy, and can often be interpreted as someone not being cooperative. Additionally, that sort of behaviour can lead newer players to believe its normal, so it can lead them astray. In general though, it is indeed quite frustrating to see...
I use voice commands a lot, but the 'request healing' one just always strikes me as quite rude - particularly as lately I'm mostly playing Zerker and constantly in need of a heal. I don't like to spam it, and be seen as that guy who can't block, or parry, or ever heal himself (all of which I do). Might just be that I'm weird though...
Unfortunately this is the reality of PUG's. I try to encourage people to run a couple of different perks to level things out, but the reality is you may get the odd game without front line zerker, or even a medic. This is where finding a good server with good regulars can really improve your games.
The difference between 0 and 5 is a single perk tier and will matter more for some perks than others. My teams often see sub-10 players added into the game (Hard) and whilst its definitely an added challenge, when its played by someone who understands the mechanics of the game in general then its not usually a problem. If I see them save for their T4 immediately and constantly request money then I get concerned. If I see their first purchase is a medic pistol ill forget about it.