Q:

Is there any disadvantage to slipping as a way to lose altitude?

I learned how to slip during my powered training as a way to lose altitude when too high and, of course, a way to track the runway center-line during a crosswind landing.
Quite a bit more emphasis was put on slipping during my glider training.  I guess the reason for this is pretty much common sense.  There is no concept of a "go-around" when landing a glider...you only get one shot.  That said, I don't practice slips often enough when soaring because purposefully losing altitude in a glider is counter-productive, except when too high in the pattern.
For this reason, I've now gotten into the habit of using a slip to descend when flying powered.  This allows me to practice slipping with the knowledge that I am guaranteed by virtue of having an engine to have the option of regaining that altitude.  Note: I usually only do this while flying solo since slipping can be a comfort issue for passengers.
This got me to wondering...
Questions:

Putting aside the comfort of passengers, Is there any disadvantage to always using a slip to descend when flying powered?  
If not, why does it seem that this method of losing altitude is not used/taught more frequently?

Update after reviewing answers
I wanted to give this some time to stew before I checked an answer.  I came into this fairly (but not absolutely) certain that the short answer was no.  I think the responses confirm this.  I understand the need some have to "warn" against stalling, but I don't really see how this warning couldn't be applied to any maneuver and really has nothing to do with the fact that you are slipping,  short of making a mistake...like skidding instead.
The conclusion I'm taking from the answers is:

Slipping can be uncomfortable to passengers, pretty much for the same reason that healing in a sailboat can be perceived as uncomfortable.
Slipping is aerodynamically inefficient.  Since the objective is to lose altitude, this would appear to be a benefit, rather than a detractor.
That some planes, in some configurations, have restrictions on when slipping is allowed/recommended/optimal.
That slipping will get you down faster, thus making it preferred if getting down faster is the objective.

A:

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.

A:

Putting aside the comfort of passengers, Is there any disadvantage to
  always using a slip to descend when flying powered?

No not really. Practically speaking it's well within the maneuvering limits of the aircraft so you are not really going to cause any problems. The only disadvantage might be a slightly higher risk of a low altitude stall due to the uncoordinated flight and presumably low speed. Apparently some aircraft can not be slipped with flaps extended 

One additional thought! Some manufacturers specifically placard their
  aircraft that slips are not allowed with flaps extended. Not sure if
  that's their lawyers or their safety folks speaking, but it is a
  reality we must acknowledge.

If not, why does it seem that this method of losing altitude is not
  used/taught more frequently?

My instructor was big on it when we practiced emergency engine outs and I received plenty of training in slipping. I'll use it on occasion if I'm really high on final but most powered planes don't have nearly the glide ratio of gliders so cutting the power is more than enough to get the plane to really come down. I would say its not used as much in powered aircraft because you have plenty of control already and you don't need to over-complicate the situation.  

