Although I like the idea, I think that the idea of a raid should be kept to benefitting only those directly involved. What I mean is, that whoever calls the raid, will get the benefits from the raid. Although slaying the raiders gives benefits too, those who choose to fight the raiders are just the natural opposition. If someone wanted to see a more successful raid they can still help with the guards, but I think the benefits for the attacking side should be kept to the raiding party only.
I may well be wrong, but aren't raiders of a neutral alignment? That would mean any power-hungry evil, lawful or unlawful, could choose to attack the raiders for gains too. Just because they're evil or unlawful doesn't mean they to work against the town if working with it produces benefits.
Just my thoughts anyway. It's still a good idea and would work either way.
Dey