As walidno1 points out, signatures may generate a lot of traffic themselves, and that can be more important than SEO. On the other hand, signatures can be absolute poison to your SEO efforts, since they generally create a lot of low-quality/irrelevant links, especially if you use signature links in completely unrelated forums. In fact, if the site automatically adds a "no-index, no-follow" to your sig links, they're usually doing you a major favour — they're SEO-neutral that way. They may not add to your "good link" collection, but they don't add to your "bad link" collection either, and it's far more important to avoid bad links than to generate a lot of mediocre ones.
Good links, the ones that really help you, are hard to come by. If, say, Google, Adobe or Apple — all with a Page Rank of 10 — links to your site, you win, period. Get two or three nines linking to you, and you win. Get a hundred thousand zeros and ones, and you're still a zero. A half-dozen fours is a whole lot better than a half-million zeros.