The problem has a relation with the behaviour of impromptu.
Like you said, impromptu takes a lot of disk space for temporal files (yes, we've outer joins and several SQL for each report), in other way if you have a lot of records you'll miss memory in your server.
We noted this problem in w2K too, and if you open the catalog in impromtu like a developer and run 3 or more reports, you'll note with window task manager that impAdmin.exe (or impUser.exe) don't free any resource when you close any report, or catalog.
You must think about a tipical scenario, Marketing Department (without any IT knoledge) need to run some reports every monday. If they note that impromptu only run one report, because at second run it doesn't work, they won't have a great impresion about the tool.
I think if cognos is selling Impromptu like best tool for reporting, they must take care in the developing of it. I haven't any experience with other (serious) reporting tool, but Impromptu gives me a bad impresion. And it's not a great tool like consultants said. And I worked with Cognos Consultant.