Not sure if it's still true today but a while ago, the number of bot accounts actually outnumbered the genuine accounts:
http://bitcointalk.org/index.php?topic=934827And having so many useless accounts on a forum can in fact make the database large. Not sure about SMF but back when I managed a phpBB board, it was definitely the case.
Most of that discussion centers around spambot accounts but farmed accounts for selling are almost certainly a part of this mix.
hilariousandco said that he nuked about 20-30 accounts in one day during the last wave of bots so it's probably fair to assume that 50 or more bot accounts were being made daily. An account farmer might have 10 or 20 accounts under his control. Bot accounts are also pretty distinguishable from farmed accounts since they tend to add spammy links to their profile or use formulaic or nonsensical usernames (e.g. I once encountered a bot that would always name its accounts by getting a common first name and appending the name of a US state to the end so for example: JohnTexas, MikeAlabama, AliceCalifornia)
And if the account makes even just one post, then it quickly becomes obvious whether it's a bot or a human.