How many years has WoW been around, exactly? I wouldn't be surprised if Blizzard is hoping to use TOR as a way to slowly start letting WoW slide off the top spot. The game had a great run, but it sounds like they're planning to start devoting a lot of time into Titan. Sure, WoW will always be around, but I doubt it'll stay their main focus forever.
By supporting it they don't back themselves into a corner by saying something like "Well they'd never be able to compete with our X,Y, and Z!", otherwise everyone would mock them for losing the top spot down the line (which will happen someday, whether by TOR, some other MMO or their own hand). This way if/when they are dethroned, they can say "We were behind them the whole time! We promise!"
Of course, this is all pure rambling speculation.