Is it "Uncool?" Well, that all depends on what you consider cool vs. what you consider nessassary. If you've built a sweet Cafe Racer or a customer chopper out of the bike, then the bags deffinatly won't go with the look. But, if you've got the bike basically stock, then the only side effect would be added storage.
I say go for it. You can get yourself a nice set of soft bags from just about any bike shop or catalog. Or you can do some searching around and find a used set of hard bags for the bike. They did make them at one point in time or another and there is always someone looking to sell something on ebay ndother such sites.