I cant see the point of alt characters in an MMO without levelling? and if there is levelling, im going to have a fit.
As far as the likes of Freelancer go, most of that was geared around having different reps, different ship loadouts and so on.
Despite being more open and lacking classes/levels/etc, I think a lot of peoples' mindsets will still pigeon-hole themselves into one thing.
With that in mind, it's much more playable with the ability to just log-in to whichever one I felt like that day, rather than having to go through hours of changing rep around, bribing people, doing missions and repurchasing all the different kit from bases scattered across the galaxy.
From a purely character-centric perspective, I can have several books on the go at once and switch between which one I read at any one time. I can do the same with my tabletop RPGs, so why not with the stories of my game characters.
There's a point where full immersion is wicked fun... and there's a point where the game just becomes hours of hassle. Immerse me all you like, but it's still a game and still needs to be fun.
Obviously this multi-char system will be open to the professional griefer-types who simply F1 into different characters stationed at various points all along the main paths just for hassling people, but it also allows the hassled to switch out and still be able to play.
Instancing will further limit the ability to grief, but I've met some seriously ingenious griefers and game-breakers who find workarounds for every mechanic you can throw at them.