The main drawback I see is....having to do business with MCI!
I just go for rock-bottom local phone service through a little CLEC in our area (Birch) and pocket the $7-8 per month in savings. Unless one of the big players can pony up 750 or 1000 miles per month ongoing (that's the key - ongoing until I disconnect or airlines stop having FF programs), I'll just take the cash.