To answer the OP, there are actually three issues getting conflated as one, and none of them have to do with why the XP-off separation remains. TL;DR version at the end.
Before the battleground split, all battlegrounds were XP-off, not XP-on. Which is to say, levelers didn't do BGs -- it was just for fun. Blizzard made the split so that levelers could earn XP apart from geared players, knowing levelers would have nowhere near the gear of veterans or even twinks.
The introduction and growth of BoAs introduced a serious power disparity between players who had BoAs and players who did not. This inadvertently reintroduced a twink-like issue in XP-on battlegrounds, but actual twinks still significantly outgear BoA-laden players, who in turn significantly outgear BoA-less players. This character (Zirise) brings a nearly optimal gearset, with only four BoAs. The importance of BoA gear for twinks diminishes to the point of disappearance by the time you reach the 70-74 bracket.
The first issue, then, is that twinks outgear BoA players by an increasingly large margin, starting small in the lower brackets, and reaching huge once you enter the upper brackets. Recombining BGs will exacerbate the gear disparity between players.
The second issue, 5.2's level scaling, won't actually impact games unless gear scales as well. Getting one-shot by someone four levels higher vs. getting one-shot by someone at the same level is still getting one-shot.
The third issue, grouping together f2p accounts and 24 XP-off players, stems from the unique limitations of f2p accounts and the 24s who leveraged the opportunity to participate in the bracket that f2ps created.
None of these issues have to do with the continued separation of XP-on and XP-off BGs. With the exceptions of f2p players and capped accounts (at 80 and 85), players who choose XP-off battlegrounds are there for the PvP. XP-on players bring additional motivations (such as honor farming and leveling) that run contrary to PvP objectives. Mixing both types of players will cause dissatisfaction on both sides: XP-off players want PvP-focused comrades and opponents, and XP-on players want non-PvE progression.
TL;DR: You're right that BoAs, 5.2 upleveling, and continued gear disparity do not support the separation of XP-off and XP-on BGs, but those aren't the reasons the battlegrounds got split in the first place. XP-on and XP-off battlegrounds serve different purposes that would conflict with each other if they combined.
Edit2: To be clear, many capped accounts do PvP for its own sake, just like XP-off players.
Edited by Zirise on 2/25/2013 11:30 AM PST