The extreme majority of account thefts on GameFAQs over the years have been for one or more of (1.) LUE access, (2.) the account is a mod, (3.) grudge against the owner (either posting embarrassing shit on the stolen account or just suiciding it), and (4.) pure entertainment (usually suiciding accounts en masse). People generally aren't going to go through the trouble of stealing an account for regular board access or anonymity when they can just make a few alts and level them enough to access whatever board.
I should also point out that account security has improved greatly since the days of GFAQs' mass jackings, both with the most popular email providers (TFA alone basically defeats 99% of attempted email thefts, which is by far the most common way to steal an account) and on GameFAQs itself. If there are still accounts floating around that aren't in the hands of their original owners, they were most likely either stolen a very long time ago (long enough ago that it would be nearly impossible to prove that the current holder is not the original owner at this point) or handed over willingly by the original owner (for money or otherwise).
Very informative, thank you. You sound like an expert on this subject. ---