It all depends on whether you use them or they use you. You should lead the relationship and be the master of it.
Can Be Useful
Agencies can obviously be very useful.
They'll get most contractors most of the work they get - so, overall, they have to be viewed as a good thing.
As longs as contractors beware of the dangers then they should be able to use agencies to their advantage.
One pretty hard and fast rule is that if an unknown agent calls you telling you that they need some information from you to help them get you a contract, it's not you they are trying to help.
They are trying to help themselves
For the most part, you should only ever take calls from agents during the period when you are looking for work and when you have contacted the agency first about getting you work.
If an agent calls you telling you that he or she needs information from you so that he or she can get you a contact then I think you can safely say good night to them and tell them not t call you again.
It's different if there is a definite job going at a definite company.
If they have specific details about a real job at a real company in a real place, and they're prepared to tell you who it is and where it is, then that's different.
The real red flag / alarm bell statement from them is when they ask you who they can take a reference from at various companies where you have worked.
You might as well save your time and put the phone down right away.
From this they hope to glean which companies take contractors and who is the person to contact there.
Agencies will get most contractors most of their work but you have to be very careful that you are using them and not vice versa.