I don't want a contract specifically stating what I am to do or not do. My job description says that, but it only covers about 10% of what I do. My title is Oracle DBA and if I was limited to what is in my job description I wouldn't be able to switch to SQL Server as the company does. I do have annual performance reviews, however what my stated goals are rarely what I actually get assigned to do. Some tasks I would rather not have to do, but others have let me expand my knowledge base in completely different areas. You need to take the good with the bad.

Having management that understands how valuable your contributions are gives you the flexibility to work on the things that best help the company and your co-workers. If management doesn't understand what you bring to the table and doesn't care or you are a poor performer I can see where a contract can give you a minimum set of tasks to keep your job. Personally I would much rather be given the freedom to learn and explore than be locked down.