I agree too but..they're called "Battle Mount" for a reason and the chance to use it in BGs/Dominion invite ppl to spend a lot of money/gold to have them.
I think that removing this chance will make the run to have them slowing down.
Why should I spend so much to take a battle Mount...without the chance to use it on "battle? Most of the end-game pvp is there, not on maps
Astral and other PvE activities don't worth it.