Ehhh, still, they EARNED those gems. Whos to say that isn't a bad idea though? You bought max dwellings, you have the kyuubi, you can no longer reborn and the Master/Grand/Diamyo levels don't seem that interesting to you anymore. Could start an alt to do it all over again, only this time with better help(since you technically beat the game).
You give people something moderated and sooner or later someone is going to end up finding a way to cheat/scam the system. It happens with everything and everywhere. >>; Just look at the function; 'You can't trade DP between alts' Obviously many have found their way around such as have I. Besides, I used to level non reborn accounts and it can be tricky being able to level without a nice supporting mount. Sometimes you'll end up buying Shukaku or the Nibi only to find out Manda and Pakkun are available the next game day. Now if you trade back your mount you probably wouldn't have enough to buy them; with this feature it could help those types of people out! Be it that simple 20-50 gems they need to get the proper mount.