Most people know who david gilmour is, but might not recognize him. That's the key distinction, and the potential problem with having the rest of Floyd come out unannounced. I certainly know Pink Floyd very well, but until recently, I don't think I would've even recognized a picture of Roger Waters. Definitely not Gilmour. I want to know it's the full floyd, so I don't spend the show wondering if that is in fact Gilmour.
And ps, Floyd ain't reuniting.