Floria

Said "typical player" can free up 16 or so slots by putting their full PvP kit into a bag, and then banking that bag and swapping when it's time for Arena matches.

Furthermore, most raiders really will only be doing 2 roles at most, whether it's tank/DPS (ferals, warriors), tank/heal (prot paladins). or DPS/heal (shamans/Ret/Balance). Bank that off-off-spec gear, and take a mage port/warlock summon combo for the less than 1% of the time when your raid make up is so screwed up that you need to perform a role that you didn't anticipate.

There are definitely the pack-everything, be-prepared players who run around with 4 free slots, and those folks will be buying these bags and subsequently filling them up. For the majority of players, a few minutes of organizing and AHing/vendoring will find plenty of space.

On the other hand, perhaps additional bank space would be nice, even after the bank buff from last year. I definitely think that players ought to bank stuff, and no one needs to tote 8 vanity pets at a time, but no one should have to trash fun stuff due to bank space being an issue.

Edit: Like you say, technical reasons are not a limitation. The real limit is UI and appearances. When I open my bags at the bank, it's a flood of bagspace, it's ugly. The makers of WoW do certain things for aesthetic purposes, and they probably don't want to have to redesign the UI to account for 48 slot bags.