I think more than jobs (because, really, MOST people will still make reservations on the phone), dining reservations won't go online because:
- Heavy traffic will prevent Guests from getting the exact restaurant and minute they choose

- Just because the site will indicate clearly what "reservation" means does not mean the Guest will read, comprehend, or agree with Disney's interpretation - and will demand to be seated at X o'clock, dadgummit, because that's what time my reservation is

Telephone reservationists can explain this and other policies during the transaction.
- People can (and therefore likely will) hack into the system and reassign desirable and hard-to-get reservations to themselves.
- Guests will arrive at restaurants claiming they have reservations and blaming that dadgummed computer when the reservation can't be found.
One more: Guests without computers (yes, they DO exist) will complain it's not fair.