I have to agree. I can't think of a very good reason not to just release the software publicly before build season. This kind of thing does not apply for building material or CAD software, so I'm not sure why it would have to for the software of the robot. It's even completely possible to code the robot in the previous year's libraries, so there really is no "even playing ground". I would understand if the reason is to make sure rookies have just as much time to practise as veteran teams, but it simply isn't the case. We've been able to practise and learn on 2012 libraries for months, not to mention 2013 beta releases being available publicly (with a link). It might be a habit FIRST has got into, and just does it for the sake of doing it.
I for one would not mind having the next year's software even a month earlier. It would let us worry about things we need to do this year, as opposed to bugs and fixes from last year. (for example, we spent hours finding a solution to bug #2
here)
Maybe there's a good reason to keep software from teams before build starts, but I haven't found one yet. Installing plugins all day is not my ideal first day of build season, and would probably give a bad impression on someone new to FIRST.
Just my 2 cents.