To win more money / medal / trips? First off, travel allowances DO stack which is kind of the whole point. Winning $600 is pretty sweet, especially when you have a paid trip. Especially seeing how Nationals is open this year, how you finish at a Regionals is irrelevant unless you finish in the Top 4. If Nationals was invite-only this year, I'd understand but why should people not compete if they get the chance to? The whole point of splitting Regionals into two this year is to allow people to attend as many as possible, and those who are lucky enough to go to two should be able to capitalize off it. Just my opinion.
I'm not trying to sound like an ass or anything, but I think it's perfectly fair that several people that have already finished in the Top 4 of a fall Regional are coming. Not only does it let you practice more before Nationals, but it also allows you to potentially win more prizes. Personally, I want to finish my last season as well as I can, and if I get the opportunity to compete again, then why shouldn't I?