If all you want to do is program a 1284p then a $10 chip and some bits and bobs and a breadboard will put you in business. But the main reason that folk get Mega's, 644's and 1284p's is lots of sensors, I/o devices, etc. and their attendant libraries. That's why the bahbot, Bobduino, and Brewtroller have so much to offer on their boards.
If there is interest in a '324/644/1284 style duemilanove, I think I have one ready to go: