I was reading Bob Parson's blog a while ago. It turns out that their support team is actually bringing in a profit (they are of course trained to be a sales team as well).
Leaving that aside, each customer that is satisfied after interaction with the support staff is a potential salesman for the company (the touted "word of mouth advertising" thingy).
The quality of the support is usually a vague, misty thing, but certain aspects can be measured and put into numbers, like the fastness of the replies (take Site5's 15 minutes average response claim), or the low number of tickets in the queue. People can more easily give weight to things when you attach numbers to them, and that can have a saying in the efficiency of the sales process.
Though, to add to your point Art, fact is that while the cost of support is a major part of the overall cost of the hosting service, it generally helps very little in closing the sale and projecting value. Most companies (for all sorts of reasons) won't go beyond stating "We provide 24/7, high quality support. We're always here ready to help you."