In my opinion ASAP :)
on every Server there is more Horde (no problem for me) but as a Allie who wants to play some PvP this unbalanced thing is pretty heavy. Horde wins about 80-90% of all BGs and 99% of WG.

I thinks it`s necessary for Crossfraction in BG because when Horde wins that much they get better gear much faster so they win more because of that ...rinse and repeat
ppl start to winning team change and keep the imbalance alive

best