Considering the fact that 9 and 10 have no arena, free battles or dlc, BEXP is a great way to keep newer units or units that have fallen behind up to speed with the rest of the party. The conditions required to earn the BEXP also add another level of depth and challenge. To the main game creating some interesting bonus objectives.
I just began playing 9 for the first time last week and love how easy it is to give any unit an extra level up. I can't really speak for 10 but I have read the system was nerfed a bit in order to punish players who abuse it.
