It would make sense for a revolver loaded only with dummy rounds though. I was wondering myself a little while ago why they didn't do that in this case.
I guess the answer is that there should be no doubt about what's loaded in a gun by the time you're ready to start using it, although if Clooney says that's what has happened on film sets he's worked on in the past then it's probably true. Agree the fact no one qualified seemed to be shouting to the media that Baldwin should have done this does suggest again that safety procedures vary massively from set to set, which does seem to indicate a bit of a problem for the film industry.
He is not the only one to make comments along those lines - several actors who have been in movies like Saving Private Ryan, Windtalkers, etc. have said they've never heard "cold gun" used on set and have always had the gun demonstrated as safe for them and/or been part of checks themselves.