...and the following situation takes place:
There's a user in your place that's pretty much the "perfect member", so to speak. He has a good ratio (not too low, not too ridiculously high, and without cheating), seeds torrents permanently, is well-known in the community (forums and IRC) for his constructive and respectful posts, maybe has even sent some feedback which helped improve a specific site feature or the tracker as a whole. Basically, he respects all the rules.
However, for one reason or the other, you check his IP or change history, and find out some interesting stuff. Namely, all the IPs before a certain point in time belong to a completely different range and country, and the password and e-mail were also different. So, in reality, he actually broke at least one of the site's rules, by trading his way in, or even hijacking the account. (It could also happen that he moved to a different country and changed his details for security reasons, but for this hypothetical situation we'll assume the account has been either traded or stolen.)
The question is - what would you do? On one side, he broke an important rule, so he should be banned. But on the other, he used the account for the better, being a good sharer and a very helpful community member, so your tracker would suffer a loss by disabling his account.