You should evaluate them on the ability to get the job done the way you
need it to be done. If every site they have ever worked on validates or
not is completely irrelevant. Can they make the site that you need them
to make? If so, then they are a candidate for the position.
You, as an imaginative customer, are not just hiring someone to bring your
sketches and vague ideas to life. A web developer should not be only a
hired muscle, he should know his job.
So if you want a really fancy page and he can make it - yes, that is a
qualification, but not all the qualification he needs. For example, you
want your page to look fine on most browsers. You don't want your page to
take hours to load due to excessive use of images. You want the code of
your page to look presentable so someone else can take the maintenance
over if need be. Etc etc.
You want those things, but you want them implicitly, ie it is only natural
that you will be angry if it turns in a year's time that those things were
not taken care of, while at the same time you are not expected to know
about them and explicitly require the developer to tace care of them.
It is the web developer who should identify these potential problems in
advance, warn you about them, and do his job so that these issues are
resolved as best they can be.
There is no way to know that a developer knows his bussiness, but having a
site that validates suggests it. It is far from a definite indicator,
however - I agree on that.