What I've read is that the yeast have now been conditioned to only ferment simple sugars (that's all that's in cider), and will have a harder time with the more complex sugars from a mash. This is why you make starters with DME, not table sugar. I'm not sure if the problem is just a super low attenuation or if they throw off flavors when trying to digest the malt-sugars.
I sort of want to try it, though, to see what would happen. It'd also be good to have a super low attenuating yeast for making malty session beers--you can keep some body but still have low enough alcohol to chug away.