Given that bot writers don't make any money off supporter bots, there's not any incentive for them to do so (unless they want to limit the number of people who have access to the bot for some arbitrary reason)
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swych
Precisely what Aria said, unless the author needs to limit the number of users, or believes the content they provide isn't quite Premium standard, that bots should preferably be free or premium. I made my Thieving Guild bot supporter due to many users reporting that a lot of the worlds doors had already been open as a result of many free users using the bot, thus requiring me to restrict the user count.