The "movie" argument is perfectly valid, you are paying for entertainment, you get one month free which offers more gameplay than any singleplayer game including epics like morrowind, then you have the option of paying for more. Lots of movies are not ready for release or should never have been released to begin with. Frankly I'd much rather pay $15 for a game that might not be up to scratch than watch something like friggen tombraider.
Your 'free' month is costing you $50.
Tombraider was crap, so I didn't pay to see it (I did eventually see it on an inflight movie). My gaming budget is finite: I get more bang for that $15 by checking out bargain bins.
Let's just say it takes them 6 months to fix things up - stability patches, hack patches, balance patches - you would have paid $125 ($50 + 5 * $15). That's quite a bit to pay to be a beta...
$15/month really isn't the big issue for me: My issue is another game being released in beta form...and that they're going to get away with it because it's Star Wars. Would you pay $15 to see a half hour composite of Terminator 3 and then $5/month to allow you to keep watching it as they gradually add a few minutes to it each month - with no guarantee that they'll even ever finish the movie?