By ensuring that your pages are valid, you make it ever so much more likely that you will not have to scramble around wasting time at a most inopportune time when the new version of a browser comes out which handles your non-standards compliant tag soup differently than the current version of the browser.

So, do you want to pay the price upfront when you can plan for it or afterwards when the fix must be done immediately because customers are complaining?

Some of us actually care about interoperability, maintainability and writing good code in general as opposed to just cowboying stuff together as quickly as possible

Why don't you bother doing your real work right the first time? As long as there's a well defined spec, you might as well follow it instead of being creative and original when it comes to implementing standards.