I tend to agree with pretty much most of what you said. Interestingly enough I manage to get an A (90) on my site somehow despite not using a CDN or expires headers. (Though I will probably add expire headers to certain content when I get the chance to research it more)
I would just like to point out that you can change these weights. By typing in about:config into the address bar and then filtering by the value ‘yslow’ you can see all the options for yslow. If you are only concerned with the weighted point values, filter by ‘yslow.points’.
Like you said, Yahoo’s problems are not your problems. Perhaps someone should setup a set of weights to change this to. Ie. if you are a small blogger use these values, if you get x number of hits use these.
And as for your comment about inefficient web pages, I do feel there is a large number of inefficient web pages out there, even on high profile sites. Especially corporate sites. Of course taking my experiences from where I work, I can completely understand how it happens.