It's an interesting idea and both the pros and cons named so far seem reasonable.
Indeed I also believe that this event could gather some media interest -- 7DRL events certainly did in the past. While media don't cover single project releases ('cause probably they don't even notice them), they would notice the releases in this event.
This is good and this is bad.
It is good, because roguelikes not known widely will profit from being part of the whole bunch of releases -- they would at least be recognized. Personally, I like this idea
It is bad, because roguelikes which do not participate in this event -- perhaps due to time restrictions or other obligations in personal or professional life; lots of conferences in September ... -- will fall by the wayside.
As do 7DRL events today, the release party event will strengthen the impression (to "normal" people) that the games and devs participating there are the core and the, well, state of the art of roguelike development, although this is only partly true.