Advantages of .NET 3.5 vs 2.0, for customers

Question

Did you ever have to persuade your customers that migrating your ASP.NET web forms application from 2.0 to 3.5 is necessary?

Why customers should install a new framework? (and sometimes a new operating system - there are a lot of Windows 2000 machines out there although support for it ended in 2010).

Which advantages for them?

• Security?
• Performances?
• Costs?

Is the only reason that 2.0 is old?

Any link is appreciated.

PS: Please don't answer that 3.5 has LINQ, it's an advantage for developers not customers