I play fiddle and have probably tended to rely too much on the dots, but I have found that learning tunes on D whistle have commited those tunes to memory more quickly, as Cluin says. And as Guest and Cluin said above, it's probably a good way of breaking the link between dots and fingering.

If there are say too many fiddles (never the case!) or whatever, it's also nice to add variety of sound in a session by picking up an alternative instument.