This discussion has been a bit more tame ever since we stopped name calling groups like AO in the midst of it; that comment was kind of unneeded.
Honestly, the only reason why I'm slightly adamant against the use of Team Preview links to how it affects certain pokemon; there's quite a selection of pokés that only run one or two movesets with little to no variance within them, and with Team Preview they've become predictable before even hitting the field, which can sorely effect their playing power in the battle that follows. This same principle also rather applies to Zoroark, who thrives on the enemy's lack of knowledge to be effective and set up, although he's only just one guy. It sort of links in with what MasterWeavile said. Tldr, Team Preview is a mechanic in itself that can have negative effects on quite a few kinds of pokés, which can affect their usefulness and strength.
That being said, I can understand why people want it implemented; aside from the problems I've presented about it, it's allowed for more prediction and switchup on your team, promoting the more skillful elements of battling, and removes inherent advantages one team might have against another by actually allowing the team against the odds to strategize and have a chance to win; it removes a lot of the luck required in a battle, luck being one of the mechanics that most people tend to spite since it doesn't promote a fair game.
That, and I can see why it'd be pointless to have both availible if one's never going to be used; a small minority may have a preference for the more unused side but that's not going to help it see much use regardless.