They are only bad if you consider them bad. Not being a smart aleck, but just pointing out that because folks say they're bad doesn't mean you can't have them in your tank. They do spread like crazy, but there is definitely more obnoxious algae to have!
I have more than my fair share of bubble algae. Yeah... I wish it wasn't there, but I've come to realize that eliminating it 100% is never going to happen either. Every month or so, I go on a cleaning spree with a hard tube on the end of my siphon hose, and scrape as many as I can off my rocks. Yeah... they pop, but if you don't minimize the population you have no chance to decrease their numbers.
I think that article melosu58 linked to is the best article out there on bubble algae.