I place a lot of weight on being W3C compliant. In case that the banner causes the validation to fail I will host the banner my self so as to pass the validation. It may not be paramount but I feel it is there for a reason and I should strive for perfection in all avenues.
Most of the banners from a NetRefer system will fail. Those are the most common ones I host myself.
As far as the programs are concerned I really don't think they give a rip. I asked one program about a widget that would not pass validation and they told me they had no plans on modifying it to work.
--
"People who are unable to motivate themselves must be content with mediocrity." ~Andrew Carnegie~