Well, the problem with tannins is that they're anti-nutrients. They chelate metals and precipitate proteins. So yes, they're dangerous to consume in bulk. As for small quantities, however, the research really looks to be out on that one. There's studies implicating them as both carcinogens and anti-carcinogens, for example. I'm not aware of any net studies on long-term lifespan changes of tannin consumption, and I imagine it depends greatly on which tannin.
On the downside: It should be noted that a lot of the research on antioxidants themselves in recent years has thrown up the question as to whether dietary antioxidants (as opposed to in-vitro) are actually good for anything or even harmful.
On the upside: some food products widely held to be healthy (both by the general public and by most scientific literature), such as tea, are tannin-rich. So clearly the whole family of chemicals can't be too bad, and could even be of benefit.