In my case, and prolly for many others, i have the network and channel i am in my sig as a way for ppl to find me (or any of our channel ops) online if they feel they need an explanation of my post or help in real time.
In members profiles there is a place to put the channel /network they frequent and various other info. Do some ppl on all msg boards abuse the ability to put things in their sigs when their intent is to spam? Sure. Is it possible that some of those are infected urls or lead to infected downloads? Sure, ditto in their profiles. When the intent is to show this is where i am if you need help, this is what i'm all about, thats a lot diff from the blatant spam we see some ppl post.
IMO, in all cases you have to take intent into consideration. When i tell someone to go to a network's website to find the answer to their question, i'm not spamming that network, i'm telling them the best place to find their answer. If instead i posted "hey, y'all come to ____", thats spam.