I'd imagine that a good fraction would actually start to tire (a bit) of the repetitiveness of describing their research hundreds of times over, and would probably want to talk about something else.. After all, does the professor usually benefit when they talk about their research to a random undergrad?
Yet, "ask the professor about his research" is generally considered to be standard social advice in academia... (after all, people love to talk about themselves - http://medicalxpress.com/news/2012-05-people-brain-scans.html ). Is the pleasure of talking about your research similar to the excitement of having the liberty to be self-centered and to talk about yourself?
But maybe professors would prefer to talk about something else - it's just that it's hard to think of a topic that they would prefer talking about, so it's simply a "safe option" to ask them questions about their research?
What I am really interested in - is this - would professors usually prefer to talk about something other than their research, if some such common topic could conceivably be found?