If the HTML-code is not validated, how big is that problem? Is it a tolerable problem for a serious website?
Is it bad if they fail? Short answer is rarely. Very little will impact your SEO and as long as the site is rendering correctly on the major browsers then it wont impact the visitors experience. Far too much importance is placed on validation by too many people. Try using the validator on major websites - Amazon.com for example fails validation and it doesnt seem to have made their site fail
As long as the site looks good to your users i don't think it should matter much. I use the validation more as a guideline to see if the site will have any majer issues.
As long as the site works on the internet, then that is really all that matters. As in my first reply and the replies from others after, many of the well known and top ranking web sites fail. As long as your site works in all browsers, I wouldn't worry about it