I think it's fine. Personally, I gain more experience (not XP) from reading nodes than from posting. I see the "XP reward" for voting as an encouragement for people to read, thereby gaining real experience.

Of course, as you say, people can just click the radio buttons without reading posts in order to gain some quick XP, but the amount of XP you will gain that way is extremely limited. And the amount of experience you will gain is zero.

Anyway, I see it as a nice little bonus. It can be exploited, but not excessively.

-- Mike

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just,my${.02}