Those power users are covering for the lack of resources and/or knowledge in a company's IT department. Excel may not be the best tool for long tail apps, but there's no arguing with its ability to quickly build useful tools. The power user that you see as spreading a virus is essentially successful as they can innovate more quickly than anyone else in the company. If open source tools gave this power user the same ability to rapidly innovate, then they should be made available to them (along with training on how to use this software).
This is what gets lots on most people.
The power users create some "nifty" spreadsheet that runs some "important" piece of a business. That "nifty" spreadsheet now requires Microsoft Excel and forces everybody in the company to have a copy if they want access to it.