Just to add onto this also. This is why I considered giving out payments, although that also has its drawbacks. Honestly, I couldn't care what the motives behind someone reporting is as long as they're not gaming the system. Therefore, I don't mind offering rewards up for users that are using the system properly, but as soon as payment is introduced I know that it will be gamed to no ends. I'm not a big fan of handing out merits for reporting, although I wouldn't critique anyone doing it as there's far worse things they could be wasted on.
I fully agree with your post especially with the part in bold. If a reward system like that got implemented I think that not everyone should be able to participate in it. It should be based on the stats of good reports, both quality and quantity wise. That way you would be dealing with a handful of members who already know what an off topic and spam post looks like. That minimises the possibility of abuse and gaming the system.