There's already an alpha testing group of some sort. If you check the devlog, you'll see Zach did some testing of .31 before its release, and I believe a few other people did as well.
Releasing versions to alpha testers will cause the following scenarios to occur.
1. Forum animosity. Perceived favourites will be selected over many others clamouring for the same sweet, sweet DF, resulting in the afore-mentioned forum animosity.
2. Release delays. Not everyone can test all the time, so while Toady may finish with a release at noon on Tuesday, all but one alpha tester may give their approval before tester X returns from his month long vacation.
3. If not all testers are needed to give approval, then the alpha testers are essentially just people who get the release first, which, I'd hesitate to say, could result in leaks, and then the meaning of 'alpha testing' is completely lost, or alternatively there's no point in giving it to them first, because there's no guarantee that major bugs will be quashed.
4. There's no guarantee an alpha tester will actually do a good job (in any sort of decent time span). I know that I didn't find many gamebreaking bugs when I first started playing the new release... It took a lot of people a lot of time to get the data needed to quash a lot of these bugs. Thus distributing a new release to a broad audience (all of us) seems like a better idea.
Just putting the versions on the forum before the main page seems like a slightly better idea, to be honest. I think Toady did that with the SDL merged .31 release.