The Mega voices were designed specifically for use in styles, and are completely useless for playing solo.
The SA voices were designed specifically for solo playing and consequently unless the style plays a solo, (Which is normally what the player does) they are completely useless in a style.
If you can find the official video (Or have them on your HDD) demonstrations of the T2, it is explained within there. (It’s also been explained at the many Yamaha user meetings that take place, where users have asked this question)
Hope this helps

Bill