Theres only one REAL reason to get a liter bike for street use over a 600 and that is you have more power on demand throughout the powerband.
If you are riding along in 6th and want to pass, dropping a gear isn't a neccisty. Basically it boils down to being able to compensate for bad/lazy riding techniques. Not that that is a bad thing. Sometimes I dont feel like dropping a gear when I really should and wished I had the extra ccs on tap.
The trade off is that there is a lot more power there for you to get into trouble with, i.e. wiping out in mid turn.
But like I have always said, a good rider will tear up most posers on liter bikes with an EX500.
Liter bikes dont make you faster, because in all reality most the squids on them couldnt push a 250 to its limits let alone that beast between their legs. Most buy it for the prestige/ego.
What is funny to me kinda relates to what Vash said though. People are always trying to make their 600s faster, with pipes, pc, etc. Like they need more power when they arent using half of what they have already. Too me if I wanted a faster bike then my 600, I would of gotten a liter bike.