Stephen:

That is true if all thermals have the same strength. In reality,
thermals have a strength (and size) distribution. On a day with a 5-
knot average thermal strength you will find thermals as strong as 8
knots or as weak as 3 knots. More working altitude enables you to
pick the strongest thermals and maintain an effective MacCready higher
than the average thermal strength. The fastest pilots (which doesn't
include me) seem to be rather good at this.

Knowing when you can step up the speed and when to slow down is the
key to winning.

Mike

My understanding of the theory is that you will ALWAYS be worse off if
you set a MC higher than the thermal strength and therefore fly faster
than optimum. Flying slower however does have several advantages, as
others have described.

Stephen