Is there any disadvantage to slipping as a way to lose altitude?
To address your questions: First, slips are perfectly reasonable for controlling descents. In some aircraft, for example, many Cessna 172s, are placarded warning against extended slips with full flaps.  In the 172 case, there may be some buffeting.

Years ago, slips were emphasized more than they are today. Of course there were more primary aircraft without flaps then.  So today the vast majority of primary trainers have flaps, and with time instructors have simply avoided emphasizing slips as much.  Consequently, the forward slip becomes more of a saving maneuver than a routine maneuver. 

Your mileage will vary. There are some instructors who put more emphasis on slips for energy management. Most instructors will emphasize them for more "urgent" maneuvers like making a field with a simulated engine out.

Addendum #1
Being the best pilot you can means constantly developing skill and adding new tools to your bag.  Slips are normally covered when doing simulated engine outs.  I also cover them first with pre-solo students when we cover other emergencies such as flap failure (asymmetric flap extension) or electrical failures with electric flaps. Slips are also helpful when there is a cabin fire, and help clear smoke faster (it's never really fast enough) in planes with bilateral windows.

Addendum #2
Slips have an advantage over flaps, in a temporal sense. You can slip, and then take it out, then put it back in.  As much or as little as you want.  Since flap changes normally mean trim changes, and in the case of the 172 there is a flap motor limiting application and removal rates, the same cannot be said for using flaps. As an example, landing during very turbulent and gusty conditions, I might only use partial flaps, and use slips for glide path management, particularly if going to some mountain top strip with all kinds of up and down drafts influencing the path. Just another thought, for someone as they consider enhancing their proficiency with slips.