The site I am most active is TeX.SX and its Meta site. As such, I can only assume that most of the rules there hold for general Meta sites as well, as there is no reputation gain (apart from Meta.SX of course)

Well, in this sense, the Mortarboard, Epic and Legendary badges with their respective requirements of 200 rep once a day, >= 200 rep on 50 days and >= 200 rep gain on 150 days are useless on a regular Meta site.

Perhaps I have some misinterpretations, so please bear with my question ;-)