For me, I'd want multiple character slots to allow me to be a law abiding trader, a scurvy pirate, and an explorer out in the far reaches of space. I can't trade in the core systems if I'm 500 jumps away exploring the frontier. Nor can I trade peacefully if there's a price on my head.
If I can't do all three without buying three separate accounts, then that would seem a little costly.
Also what about families where a parent plays the game and there are 1-2 kids who also want to be able to play?
If they're well off, all have separate PCs, and all want to play at the same time, then sure, they should buy three copies of the game. If they're not that well-off financially though, and have only one shared computer, then it makes much more sense to have multiple character slots per account.
Maybe allow unlimited single player accounts, but have only 5 online ones?
I also think that someone elses's idea of not allowing you inter-player trade until your character's a minimum age (say 48 hours old) is a good one. It means you *can* help friends / family members / etc, but you're going to have a fairly rubbish time trying to exploit the system.