I'll produce error messages for missing but required input, but that's it. I want my programs to be used however the user needs it, including in shell scripts and cron jobs. I can't rely on any particular use.

Programs that insist on prompting for things are much more work to automate. You end up with entirely new languages like Expect (or the Perl equivalent of Expect) where you have to fool the computer into thinking you are a human, and you have to know the prompt strings and other interface bits. I typically want to spend as little time as possible with the computer, so I don't typically don't design programs that make me actually be there.

That being said, some programs might have some sort of flag to turn on "interactive mode" or some such. I have no problem with that sort of interaction as long as it isn't the only sort and as long as I don't have to work to turn it off. :)