I have a few theory's
1. To make f2p pvp unenjoyable........(Wait!)....... in hopes that it pushes new comers to sub, as this isn't what endgame has to offer...Cough!
1a. Since going unlimited f2p and splinting the brackets, Blizzard would of noticed an increase in server activity in the lower brackets, and because f2p's being the bigger population static, don't generate revenue. meaning their profit margin from new subs from Trial accounts would be in the negative.
2. Back in the day I remember entering low lvl bg's and seeing players with a red status, almost impossible to hit and could most times solo an entire grp. which brings me to this, now that hit and exp are being removed, there is no need to have a split in the bracket plus with the introduction of gear scaling.
3. Going back to brackets with increments of 9 might mean better use of their servers and or speed up ques times across the game.
I understand that trials accounts have big restriction's to combat gold farmers ect, but in my opinion and i think Bisonpower quoted this, that they should unlock and improve the cash shops as this is the trend for most upcoming MMO's.
And by that I don't mean the p2p to win one's, but more like a GW2 and Wildstar model. Spending money in those games does give advantages, like buying exp boosts and gold, but people who don't fork out irl $ still have access to these items., they just have to earn it in game.