User Profiles in Testing

I just finished reading Joel Montveliksy's blog called:

5 Ideas on how “User Profiles” can Improve Your Testing

I really like the concept.

What struck me most is I asked myself, "how well do I understand the users of the software that I test?"

Joel proposes that each user profile should contain all the personal and professional traits that are relevant to the work this person does with your software. That includes describing how the user works, what their goals for using your software are, what annoys and frustrates them, what delights them, and so on....

How many of us can sit down and describe our users?

Shouldn't these user profiles, whether we use them or not, be created and validated by others? To ensure that our assumptions are correct?

Shouldn't this be absolutely essential if our testing is as effective as it can be?