I'd say one advantage that's easy to sell and take advantage of is that it's cheap. You can do some small projects using MSDE, gives you a chance to show what you can do with the platform and point out how you can easily upgrade to the real server edition if/when it becomes necessary.
More than that, isnt your real problem selling them on technical solutions at all? Lots of people like the status quo. Do some small projects to show it works that solve real problems, get them interested and used to the idea.
Andy