...They can combine realms with opposite faction-imbalances and similar populations to artificially create a balanced whole.
That only works for the time immediately after the merger. There's no way to stop more players from creating more characters in the dominant faction and imbalancing the realm yet again.
Yes, putting together connected realms does take some thought, but they are doing it anyway. Faction balance is one of the factors they can take into consideration, to try to generally even things out.
They can also use CRZ to dynamically faction-balance each zone. No hands-on-effort whatsoever, it's a basic load-balancing type of algorithm. Realm group (with connected realms/current CRZ) has, say, 30 at-level Horde and 5 at-level Ally toons, and 3 Horde level 90 toons. So, the next 25 at-level Ally toons and the next 3 90 Ally 90's who enter the zone, on a compatible server type (RP or not, PVP or not), will be CRZ'd in with them. As people enter/leave, or log in/out, it can adapt to them.
I'm not saying that Blizz will do this, or that it is necessary. I'm simply addressing the claim that Blizz "can't do anything about faction imbalance." They already have been taking steps in this direction, I've actually run into Ally toons (from other servers, of course) here on Illidan. My Stormrage shammy has seen lots of Horde toons (likewise), even though neither server is currently connected to other realms.
I leveled two Ally toon on Tichondrius when it was a relatively-balanced server (not perfectly 50/50, but decently close). I had a lot of people try to engage in WPVP with me, but usually just putting my PW:S or Sacred Shield was enough to discourage them. I quasi-grouped with Horde a lot while leveling my Pally, and on Isle of Thunder.
Most people only do WPVP with targets who are much lower level, much less geared, or afk (ie. camping flightpoints). The mildest of resistance is usually enough to persuade them to seek another victim. This applies regardless of whether flight is enabled in that particular area.