Its been a while, but I recall that you -do- need to make an account - it gets created when you make your first purchase. But having to send people off to another site (where they get to reenter all their address info -again-) is a PITA and is not as 'professional'. Its getting more common... but if you made it through the hassle of getting a merchant account there is a bit of respectability there.

Also, we have encountered institutions (schools and businesses mostly) where there is strict policy that they may not use paypal.