Same situation - did a rotation at the home program that had no residency program, and then started aways. For me, not sure how much it helped. The letter was well received, but I think I should have done some sort of project/presentation. B/c it was my mentor and a laid back rotation, I didn't get much out of it. I agree that it may not be worth your time, unless you needed to learn the vocabulary and the basics of the field. It seems like you don't need to do that.
Anyway ... I'm furloughed until September. Didn't realize Step II CS would be necessary for my PGY-2 license. It's a nice little vacation, but I was very excited about starting. So, I orientated with the department and am now off duty. My chief resident joked that this blunder cost me $60,000 ...
-S