the rounded corners seems like a weird gripe to me. You're losing what, about a millimetre in the corner? Have you ever seen a ship landed so perfectly in the corner that it would make a difference?
Well considering you can set up your ships that way - you could in theory START your ships in the corner, and thus be off the map (Which I'm certain nobody would ever actually try to disqualify you for...),
But also, the biggest reason is that if you line up two mats together like they suggest for epic play, the small bit you lose becomes even more pronounced and then is right in the center.