When we have fresh new people in congress every now and again it lessens the hold that special interests, big corperations and the rich have on congress. When money is involved in the legislative process a congressperson is inclined to follow that cash because it may help them get donations for their re-election campaign, rather then helping out the average American who doesn't have the lobbying dollars or power to influence a vote.
This causes legislation to be passed that does nothing to help the middle and poor classes in this country and everything to help the rich get richer. With term limits I'd say atleast 3 six year terms for a Senator equaling 18 years and 9 two year terms for a House member also equaling 18 years is a good and fair place to start. We can begin to lower that further at some point in the future but we have to start somewhere so we don't have career politicians representing us for a life time.