A teamwide slowdown is definitely a team disadvantage, since it can be filtered to team, it would have a very visible effect on gameplay. Or is there no difference between the run speeds of a medic and a soldier?
Griefing happens, you can't always address it; the flag issue in a/d ctf is more of an issue than this. Slowdown however, is not necessarily fun. A one-time per quake -15 hp quake for the opposing team, for /some/ player-induced action, would be interesting. This could be passing through a difficult area, causing an enemy player to fall into a fault, etc.
If the reinforcement encourages faster gameplay, then there is a definite use for it. You could even put intels into play, with one on each CP; sometimes players can get to a cp, but not fully cap it- a new option would be to grab that intel and return home, synching with your team when you're about to drop it: say returning the enemy's intel to your base does 50 damage to the opposing team. "I'm about to return; push!" And hit the now-defenseless team. You could even have it do 100 damage to enemy buildings, or buildings on their side of the map. POOR ENGIES! But quakes destroying buildables would be a definite stale-breaker.
edit: I kind of have a boner for the buildable-hurting quakes

make it happen buddy