One has to keep bb height within reason but I can't say that there is a correct bottom bracket height for mtb's. Not every rider likes 29" wheels or single speeding or full sus or 27 gears or carbon hardtails with 80mm of travel. The same goes for bottom bracket heights. Some riders, depending on one's riding style and the terrain their riding on might like and 11.5" bottom bracket height whilst his riding buddy might be better suited to have a 13.5" bottom bracket height. Use your best judgement. I think it's also silly to say that a bike is going to be impossible to ride after slackening the headtube angle and raising the bottom bracket height of a bike(exactly what happens when you put a 650b tire up front). Isn't that what adjustable travel forks do to a bike too(yes)? It can't be any worse to tuck 650b's into a bike that it is to tuck a fork that's adjustable from 85-115mm of travel or 100-130mm.