In some cases it's obvious that the estate agent has to show potential buyers around. (e.g. when the vendors live the other side of the country from the property that they're offering for sale and don't want to travel to every viewing).
At other times though it really ought to be down to agreement between the agent and the vendor. e.g. the estate agent might phone the vendors thus: "I'm just calling to confirm that I've made an appointment for Mr & Mrs Bloggs to view your property at 3pm this Saturday. Would you like me to be there, so that I can show them around myself, or are you happy to answer any questions that they might have?"
As a potential purchaser, I'd rather be shown around by a vendor than by an estate agent as there are some questions that an estate agent might not immediately know the answer to (e.g. "Can you tell me which school's catchment area this house is in?") and some that he couldn't possibly be expected to help with (e.g. "Do you get a lot of noise from people coming out of that pub down the road?")