I think that, like anything, it's fine as long as it's not abused. There's nothing wrong with a line or two giving a link to some guy's (or girl's) want or sales thread, or a special knife they are looking for.
It's fina as long as it stays at a line or two, not some huge banner with reams of text. That would get a bit annoying. Is there a size limit for sigs?
And if it is abused (not that I think it is atm), I would say to take it up with the individuals concerned, not punish the majority for the sins of the minority (Like our enlightened UK government seem to do all the time)
Just my humble opinion...