Seriously... I don't want to discourage anyone from validating their
site; however, unless it's REALLY broken, we're likely going to be able
to spider it pretty decently.

It's more important -- from a Google-friendly site perspective -- that
your site adheres to our guidelines and is broadly accessible
(serverwise, browserwise, platformwise, etc.)

Being more specific:  I'm betting that in the vast majority of cases in
which folks have indexing or ranking concerns, the core issue is NOT
that their site doesn't perfectly validate.