Again, the Hero isn't a Sprint product, it's an HTC product. Naturally, it would be HTC's job to test their product, and their apps (yes, including the Messaging app), and Sprint's job to test their apps on that product, as well as any other Sprint specific areas. It simply wouldn't make sense economically for Sprint to test areas HTC should have already had tested. Not to mention it was HTC's first Android phone on a CDMA network, there's bound to be quite a few bugs right off the bat.
If HTC had done their job in this case, then Sprint wouldn't have had any reason to test the messaging app.
You'll notice that the Sprint apps actually work pretty well. They're pretty much worthless to most people, but they do work. Why? Because Sprint tested them, and I believe that is what they are working on now. Porting their proprietary apps over to 2.1, making some modifications to those apps (and perhaps adding new ones), and testing on their signals.
If you don't like the damn phone, then why are you still using it and complaining about it?
Besides, it's an Android phone. The thing's designed to be hacked, to be customized, to be optimized, by other people than the manufacturer/carrier.