It depends on your purpose. They exist, so they are good for something.
First, you need to understand what are they. Please see my past answer:
What makes static methods accessible?[
^].
One important application for static members is the
singleton pattern, please see:
http://en.wikipedia.org/wiki/Singleton_pattern[
^].
Here is the good C# implementation; you can figure out the purpose of static:
http://csharpindepth.com/Articles/General/Singleton.aspx[
^].
Please see my article for a very advanced (and funny) application:
Wish You Were Here… Only Once[
^].
Static methods are used not so often; and they should be used with care when they also use static fields with access from different threads. In my article referenced above, you can find some valuable criticism by Paulo Zemec provided to fix a mistake I made.
At the same time, there are many functions where using them as instance (non-static) once would be a mistake. If they don't access instance data and are instance methods, it's a certain performance leak. FxCop, for example, will report it as a mistake.
[EDIT]
Two more important uses of static methods:
Operator overloading:
http://msdn.microsoft.com/en-us/library/aa288467%28v=vs.71%29.aspx[
^].
Extension methods:
http://msdn.microsoft.com/en-us/library/bb383977.aspx[
^].
—SA
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