Term limits lead to institutional knowledge and skills being concentrated in unelected staffers and lobbyists. Effective legislating involves building relationships, negotiating skills, and deep subject matter knowledge in at least some areas.

Yes, we have tons of bad legislators: some just not good at their jobs, some actively harmful (leaving that vague on purpose—I think all partisans can agree that they exist, even if we disagree on who they are). In theory, they can be excised via the ballot box. However, we don't want to kick out the good ones just as they're getting to be their most effective—not only do we lose their direct skills, but we lose their ability to mentor the promising up-and-comers.

I place more blame on the way we do primaries and general elections: in most districts, the only thing that matters is the primary, and that produces some truly rotten results.