I completely agree that IE has plenty of rendering bugs. Your first post made it sound like creating a standards-compliant page is almost impossible because of the issues. I thought I'd offer another opinion that, despite these forced work-arounds, IE supports HTML and CSS well enough for standards-compliant pages to be very much possible.
I always make my sites w3 compliant, and it doesn't actually take any time, you just have to develop the habit to write valid code. Of course, it's easy to make your site w3 valid even though your html design is a crap (use of tables to form your layout, inline css etc), so the mark 'this site is w3 validated' doesn't mean much to me.
I use it as a de-bugging tool. I would never rely on having a "validated" site to tell me whether or not my site looks "right" in a range of browsers. Valid coding does not equal logical, right coding. Especially when it comes to IE.. it's always fun working with IE. Also.. If I test the site and it's just a bunch of missing alt tags on spacer.gif then well I probably won't bother.
I try to keep my sites w3 compliant. For example not having alt tags will be noticed by dialup users.
It's not true that a site that validates relates to SEO, it has little or nothing to do with it. What matters is semantics, and there aren't any validators for that. If you don't put a h1 tag in your code with a sensible title and keywords you'll likely score a little lower. If you put h1 tags everywhere you'll also score lower. The point is that you should use your tags what they're meant for, so the spider will understand the purpose of your data a lot better. If you use a font tag for your title instead of h1 the spider has no way to determine of the data in the font tag is the title or not.