Way back in the day, Blizzard had a style to prevent verbal griefing, targeted camping. Once you made alliance or horde on a server, you could only make characters of the same faction on that server.
Players got used to this policy, no matter how odd it seemed at the time. If they wanted to try the other faction, they would simply need to roll on a different server.
Later Blizzard did away with same-faction servers - but only because they were promoting added-value services (paid-for faction transfers, in this case).
I think Blizzard should use the original Vanilla policy of only 1 faction per server. This is how the game was built, originally. To prevent someone logging in YOUR faction to bitch you out for killing them. To prevent someone from monitoring your movements, so they can switch back and ultimately farm you, all day. To prevent guild monitoring ... many more reasons.
IMO, it would be very un-Vanilla-like if Blizzard allows multi-factions per server. They may try this to make some $$ via faction flips, but I will still frown upon it.
These are my thoughts ... what are yours?