It's more likely that Apple/AT&T/O2 etc. have agreed on this to make more money from text messages...and also it does make the whole experience more seamless I guess.
My wife went text-crazy with her iPhone, because it was so easy. She would send out these long texts in lieu of actual phone calls. Of course, we weren't keeping track of how many we sent, and didn't think that many of these texts were actually split into 2 or 3 messages.
The up shot was a large overage in our first bill for text messaging. I bumped the limit to 1500 messages, and so far have been ok.
FTR, I'm not complaining. One of the interesting things about the iPhone is realising just how much it changes your way of doing things. This was one of them, and now I've adjusted our plan accordingly.