I think it is unique to each business. As Rob said they have a lot of small customers so webmail is popular in their database. On the other hand, we have never sold anything to a webmail address. If they are serious purchasers, they always provide a business mail. So in our case, we do a lot to clean up webmail within our database since we are paying by the email in Eloqua.
So, I guess the right answer is "it depends".