I am affraid, however, that this thing will encourage people who are bad at spy (and know it) to play it and mess things up a little for the team they're on.
You can't possibily mean that. How are you supposed to improve your gameplay if you don't actually play?
And we're probably all going to farm the achievments with some friends on empty servers anyway.
Yet they do ruin gameplay for other spies, bartg is right. To be honest, spy is the only class who's play can be totally ruined by teammates. Scouts and pyro's in particular...
Only class?! What about a soldier bumping a pyro right in your face because he can't control his hopping? Or a heavy that keeps dancing in front of your scope? Or a demo stealing your ammobox in setup time. Or a pyro that keeps blowing back people completly messing all your shots on the target. Or..and I could go on.
The spy feels griefed only because it's hard for them to wreck other people's play. And that is true, since the spy is mostly behind enemy lines. The only thing he could do is draw attention and that's always positive for the other players.
As for the scout thing, don't say stuff like you don't know how to play scout. You come from behind of you flank the enemy. You don't draw their attention face2face and hope their aim is crap. Though that would certainly put the spy trying to close up from behind at an advantage.
Pyro's and scouts tend to run circles around their target. Which really helps when you try to come up and backstab someone. The point is that on the pubs the scouts just rush by you and start firing at the enemy or they start shooting from an ineffective distance. They draw attention to the rear. Which really sucks when you could just have stabbed 3-4 people in a row.
The sniper can get annoyed by somebody blocking his headshot, but he is relatively because he is standing at range. Thus he doesn't die from it. A demo stealing the ammobox won't result in a premature death either. The pyro blowbacking will actually make it a bit easier to aim at the target...
sod it
The thing I am trying to tell you is that other classes usually dont get gibbed when someone is getting annoying. The spy does, because the whimpy gun isn't gunna save you against the almighte bonesaw, wrench, flames, spray, splash
(patiently waits till someone goes nitpick this post to point out something totally uncovered by this post)