Yes I think most people with their websites in the siggy would prefer to have their signature viewable always. I totally agree with Cricket, the more exposure our sites get the better.
Spambots are just a consequence of having a website, if they don't harvest it from a forum they will get it somewhere else. It is a pain but I try to get around it by using free email accounts and not my domain email, that way if the spam gets too much I can dump the email address and get a new one.
Besides, sometimes people with art related businesses browse art forums and contact artists with their offers, I don't mind getting that kind of spam.