ok i wasn'T there in Vanilla when XP was gained by throwing out marks.
but i had friends who were there and you know why blizzard removed it ?
simply said, the AFK IN BG ALL DAY LONG going on. people we're using BG to get levels while they were going to work. my cousin was one of the people who done it, it was simple, he had an addon that would auto-queue him auto-accept the invite when it popped and he had planned a way to make his mouse move a bit every seconds or so. thus he would never become AFK in games.
he gained like 20 fucking levels just by not sitting in front of his computer.
that's why blizzard removed the thing, i really really doubt them to be doing the same mistake twice ! though the gamne has changed and now people are able to kick people out of the BG or reports AFKers, it is still going on to this day for the honor, so now imagine if they bring back the old way of gaining XP in BG, it is madness i tell you !
as been mentionned already, the gaining levels in BG been heard about like zillions of time each fucking patches. we still not having it, so no it won't happen !