Well my point is that people are unlikely to block signatures because signatures are often very useful. However if too many people have the same advertisement on their avatar then people will start to disable avatars.
Avatars can be useful too, but if people are annoyed by signatures or avatars they'll probably just block them anyway.
It is easy to measure how many people see a billboard or a TV advert as you can measure how many people pass a certain location or view a tv show. It is much more difficult to measure how many people see a certain user''s avatar which makes it difficult to price how much an avatar is worth
You can say the same here with the number of hits the forum gets - after all that's why signatures are so valuable here from an advertisers standpoint - but like TV adverts or billboards viewing figures or views are often wildly inaccurate and are estimates and as such don't always translate into sales or people even actually paying attention to them. As with any advertising you're just buying the time and space and hoping it gets you sales or at the very least gets your brand out there.
There is a huge difference if you would ask me. Signatures often contain useful information and further references for such a service, while on the other hand, avatars contain only an image which is solely used for representing a brand/service. With this links and texts, a user can already know what the service/product is all about, thus making things easier on the user's side. Signatures imo is a better advertisement for a product/service than an avatar.
I'm not doubting it's better but an image can also contain much info and is very valuable imo. As I've said before it's essentially a mini billboard and you can utilise the PT and weblink to direct people to your website as well.