Yamaha can do some pretty stupid things some times... I personally think they bit the big one when they left out the battery option on the 640/740, and you're right in that the case was originally molded to have this. I currently have the PSR-550, and it's not really a bad keyboard, but if you can afford it go for the PSR-1000. That board is better than both the 550 and 640. One HUGE plus is that the 1000 has in internal sequencer with extensive editing as with the 550 and 640 you have a REALLY basic 16 track direct disk sequencer that doesn't really offer much to the user. Another option you might want to consider is Yamaha XG works.... With this you can use the 550 with a computer and have a way better sequencer. Also I think there are many reasons to get the PSE-1000. Sounds a little better, louder speaker system, great large screen like on the 9000, voice creator, which the 640/740 seriously lacked, better sequencing, more storage space for user styles, and so on.. It's an all around better choice.. Plus it's price is fairly decent too..