Forum Moderators: Robert Charlton & goodroi
If you did pass "juice" it would be the exception rather than the rule. Many ad campaign URLs are disallowed via robots exclusion, and in addition, most use "unfriendly" redirects which prevent any organic benefit. And of course, Google does make an effort to discount links with a purely financial relationship between the two sites.
Worse still, many ad programs use images created via javascript and/or within iframes, which make it unlikely that a search engine can even interpret the code, let alone follow a link and give it credit.
That doesn't make it impossible for a banner ad to pass benefit, but it would require specific steps to make that happen. Added to that, image links are less than ideal even with an alt attribute, and most banner ads I've seen do not use this attribute at all.
If its a smaller independant site, when you are negotiating a price, demand a proper html text link somewhere else on the site if you can. Some will do that, most will say no but its worth asking.
I run ads on my forum (directly negotiated, only for companies in my niche) and they get a block of text including a link or two if they want - and the links not only pass juice, but they pass good juice. I see no reason to play patsy on something like that - they're good links, hand evaluated, and I treat them as such. OTOH, my prices are high enough that no one's going to advertise there just for the link juice :).