To squash any rumors on this point, the latest update very clearly said that you may only attend the event at the time you were invited for, so only once and only in one day.

As to worth, had they invited all that registered to Day 1 and then only some to Day 2, it would imply less importance to Day 1. But, UAL did cull the herd and sent out a few TBNTs as well, so if you got a Day 1 invite, you did make a cut to get there, but something less than the Day 2 cut. I would go if you were invited and can make it without calling out sick or something. I think it means they are looking at you in the future, and you could end up getting some type of black ball if invited to Day 1 and you don't show up. At a minimum, you have to email and decline the invite which is better than a no show.