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Why? Being good at math isn't necessarily an indication that you'll be good in history. Perhaps you're an awesome helper in math but mostly need help in history.
(We do track an overall OpenStudy level, it's just not emphasized at all at the moment.)
I would love to hear your rationale -- we waffled a bit on this when we were first devising the points system.
I don't think that your XP measure how good you are as a helper- I've seen some really smart people giving educated answers, and they're only a 'phyte, and I've seen people who aren't really that advanced in a subject have high ratings.
I think that the XP is more of a gauge of how long you've been on OS; therefore also used to it you are and how well you know its quirks. That doesn't change from group to group.