Most quality forums now have a post count before you can add a sig file. Joining 100s then makes little sense. Also doing things which go against the norm is always easy to detect and filter for the search engines. Algorithms use averages to determine legitimacy. Say I and most SEOs work in 5 forums. The search engines use the data as an average to measure others against. And you want to be the best SEO so you join 1000 forums Google uses the baseline measurement of all SEOs and compares your actions to the averages. Your work sends a huge spike in the algorithm. Google knows this is artificial, and done to effect rankings, and so they would filter your forums signatures from the score and you will have wasted your time and energy. More facts for your thought process. Peace
Traffic wise I haven't goten that much from my signature here on DP, but I've moved up in the SERP's. This indirectly gives me more traffic, so I guess you could say the signatures could give you plenty more traffic!