html validation: exactly what is it good for?

Hi
I have a question that crossed my mind, and i wish to get help from you
guys: HTML Validation- what is it good for?
I try to find the disadvantages of a non validated site against one
validated, and let me try to explain what i mean.

If i build a website, according to his declaration - let say Strict XHTML,
and its answering all the roles of Strict Xhtml, but its not valid cause,
for example, i haven't used "" to surround values, or i haven't closed
single tags using Slash (< />), but other wise i did everything by the
scheme.

 What can be the damage?
If i open the site and see that it works on my target browsers, why i need
the validation for? what do i benefit from it?
It says it helps me improve my SEO, but i see all the first results at
Google, for lets say "Forex" keyword, and none of them are valid.

I assume the main price is time rendering cost, but what can be else?

Thanks a lot.
Shlomi.A