The thing about these big sites is that they have an army of programmers to guarantee that the site keeps working.
Do you? Are you willing to spend your money on that?
The problem with not following the standards is that you have no guarantees of how your site is going to work in tomorrows browsers.
Then there are other issues, like accessibility, or enabling automated tools to parse your site. Whether this is search engine web crawlers or sites aggregating information available from microformats, or screen readers for blind people or any of the dozens of other tools that need to be able to read your site, you've no guarantees they'll be able to parse your site if it's a random tag soup.
Then there are the tools that you yourself us. Will jQuery or other javascript libraries be able to figure out your weird non-standard DOM? Maybe, maybe not. Will next week's version? Who knows?
And finally, what is the cost? It's not that hard to write compliant HTML/CSS. It takes some practice to figure out some of the CSS tricks to avoid relying on deprecated or nonstandard HTML, but once you've figured it out, it's just as easy to write as your typical sloppy nonstandard code.
And of course it may even make things easier, because it allows you to meaningfully use tools like the HTML validator when debugging. It's hard to use that for anything useful if your site contains 50 HTML errors in any case. Which one of them, if any, is related to the problem you're trying to fix?
