It's a technical measure, not a marketing one. We're going to open up pieces of the product to general use as we smoketest and validate their performance. The join page is more or less there to be served Very Very Fast (and still let people be notified when stuff opens up) more than it is to lock people out of content.

This iteration is a ground-up custom build rather than an iteration on Wordpress, so we're being conservative with our testing. It won't be that way for too long.