I find that this is really something you should not worry about after you have mastered basic skills of HTML development. The sites that offer validation or optimization tips use code checks that expect strict compliance with all of the standards however browsers can display sites even with some serious errors.
There was a time when IE was known for closing tables and divs and other elements when other browsers would choke on a page. In these cases it is not such a bad idea to shoot a page at a validator site just to find where your error is. Then after fixing that problem or open element double check it in a few different browsers to make sure your site works well.
Now HTML validation should not be confused with validation for useability for the visually impaired.
Sites that require validation for these users … specifically government sites should take care to make use of a validation service or provide an alternative text only / low media use… version of their content.
Another reason to validate may be for bots that index your content.
Google is getting picky about the content they index and asking sites to include a number of items within elements that will make their job easier. They also have problems with sites that take longer to load.
However do you make your content for bots or humans?
I think you have to justify any modification of your designs when writing for a bot rather then a human. If you can implement changes easily maybe by using php to grab alt tags from a title or inserting reference information into tags then that is fine.
If you find that you are taking more time complying with demands rather then generating content then your job is wasted.
So …. Are HTML validation sites worth their use?
Maybe… but don’t get hung up on it.
Your content will get indexed… Your content will get read by your visitors..
But if your boss wants to give you overtime.. or your clients want to pay you double.. then go for it..
Otherwise … take it down a notch.. use good design standards and make your sites work for your visitors.