New Impellar. . .

I am just curious to know how often the housing goes bad for the impeller when the impellar goes bad. I talked to the boat shop about a week ago and told them that I thought that my impellar was bad. I said that I didn't want to have to wait to get the parts in once they finally got to my boat if it was the impellar. The shop manager said that he has impellar's for my boat. Anyway, they finally got to my boat yesterday and told me that they would have to wait for a couple days to get the housing for the impellar. I have read a little about the impellar and others have said that it was common for the housing to need to be replaced if the impellar goes bad. Just how common is it though. Am I overestimating how common it is or is the shop I took my boat as idiotic as they seem. There is much more to the story that makes me believe that they are idiots, but thats another thread that I don't want to get into. My boat is an 89 inboard/outboard with an OMC outdrive.