I think we may still be talking at cross purposes. I’m talking about making an ad appear directly below the content on a single post or a single page. I don’t care whether it appears on archive pages or on a “blog” page (ie. a list of posts). It doesn’t even matter if it’s not a widget because I won’t be changing it – I’d be happy to paste the ad code directly into the template. Is your solution talking about the same thing?
I think the main point I’m trying to make is that not everyone uses the “posts” as a blog – people use WordPress in many different ways nowadays – and it’s quite common to use “posts” as the main content, because of the fact you can use categories and tags with them. In my case, it’s a bit disappointing to find a blog with so many widget options, then find I can use them on only ONE page in one of my websites!
You’ve given me the solution, but it just seems like it shouldn’t be necessary to ask. Perhaps making a decision on where widgets should and shouldn’t be allowed is something you should leave to your users, rather than assume that “posts” MUST be a blog and therefore can only have one sidebar and a coloured footer?