Well ... also you *must* give a table. Another way to do what you want would be to derive real_users and pseudo_users from users and define no new table for either type, adding columns to your shared user table instead. I find this latter inability to define a type without additional storage to be more annoying but you're not the first to make the suggestion you're making, either.
I think both of these possible changes are on the table for OpenACS 4.7, but not 4.6.
