This seems to be somewhat of a PR move for them since this is not much more than a free trial which would require a paid account if you expand beyond 50.
However for smaller groups - it's not bad. I am at a loss to really explain to you what the benefit of paid vs free is though.
I know the paid version adds external contributors (which we found really didn't work because the external contributor search was severely limited).
Other than that I think the 50 user is actually a limit because in the prior ""free' you could add unlimited users with very few indications of any limit other than I know you couldn't make some technical connections on the free version to directories, etc.