I have seen ditrs used on a countertop, on the ditra site of course...lol. Does anyone actually use ditra on a countertop?

Brent,

Saskatchewan

Yes

Is there the potential movement to justify the extra cost of ditra?

Not as much as a floor installation, but regardless of the size of counter top... there still needs to be a membrane between the tile whether it be cement board, Ditra or mortar bed. Tile to wood is a gamble in any application relating to tile or stone.
The good thing about Ditra is that it is also waterproof and may keep areas around the sink a little drier by inhibiting the absorbtion of water into the wood work.

I can understand the need for a membrane. That said, would kirdy membrane work for this application?

Alot depends on the cabinet and counter top construction.

The goal is to get some uncoupling. Thats why the underlayment needs to be cement board,mudbed or Ditra.
Kerdi will not provide that function.