In large corporations, there are often webapps that depend on ie6's buggy behavior, and break with anything else. And it may not even be under the company's own control --- I know of one SaaS player in the financial space that has a major bank pinned down to ie6, even though the bank would like to upgrade.

Chrome Frame is one way out of these situations, but convincing IT that it's a good idea is a struggle at best.