Shirl:
The difference is that when the longitudinal axis of the airplane is not
lined up with the runway centerline (forward slip), there is much more
drag.

Jose:
This is true of a side slip too. I suppose if you want to lose
altitude, you cross your controls more, but again, the =airplane=
doesn't see the runway, so the =airplane= can't tell the difference.
It's just cross controlled.

It's true that the action of the pilot is the same; however, in a
forward slip (for loss of altitude) it is cross-controlled to a greater
degree so that the airplane is actually coming down sideways (but in
line with the centerline) so as to expose more of it to the oncoming
wind. Yes, there's drag in a side slip (for crosswind correction) too,
but not as much, and the whole point of the side slip is to keep the
airplane lined up with the runway, not to lose altitude.