I get told it's very common.... I guess I have a conscience and make a time to go do a quote if I have organised one, and ring if I am late or can't make it that night. I think it's poor work ethic to knock back work...even if too busy all you have to say I will do the quote but it cant be done for x weeks or months.
The last few months I'd say every couple of customers have other contractors who have no turned up.