The key is the unpopular site - small businesses, for instance - that want to compete in search engines but will never have thousands of visitors a day.
Standards-compliant websites do not necessarily make for better SEO. But the practices and culture around them do.
Accessibility generally results in improved SEO simply by 1) increasing the placement of relevant text within a page and 2) making the site more accessible to search engines. Things like alt text go a long way.
As for download speed, you're absolutely right. It's a matter of data size. But standards-based design lends itself toward smaller pages simply by removing the need for repetitive code like
It's not the standards that make it work well, but the benefits that come along with the journey towards those standards.<font face="Arial,Helvetica,sans-serif" size="2">
Nowadays, if a client isn't willing to let my company develop an accessible, standards-based solution, he isn't going to be my client. I just won't waste my time on them.