Does the author(s)'s intention matter?

So I've been in discussion with various persons for awhile now, debating what effect authorial intention should have on 1) our interpretation of a text (book, movie, song, anything created by someone who is capable of intentional thinking) and 2) our evaluation of the text. So as a simple example of how an author's intention or misintentions might give us pause when considering what might otherwise be evaluated favorably, consider the line "Desmond stays at home and does his pretty face, and in the evening she's a singer with bang" from the song "Ob-La-Di" by The Beatles. Paul McCartney did not 'mean' to say that, but rather "Molly stays at home and does her pretty face" (as he does in the preceding stanza). Should we credit Paul McCartney any less? Related, should we value the song any less?

As to how a text might be interpreted differently relative to what authorial intention we impute, consider Hamlet's love poem to Ophelia:

Doubt thou the stars are fire,
Doubt that the sun doth move,
Doubt truth to be a liar,
But never doubt I love.

A bit hokey, no? However, was it supposed to be hokey? Considering 1) Hamlet is a bit on the overdramatic side and 2) Shakespeare's written a ton of wonderful poetry, it's difficult to interpret this text-within-a-text as anything but intentionally hokey. I'll spare you all the wonderful deductions I've made in light of these observations.

So what's the point?

Apparently, not even authors in collaboration are always on the same page when it comes to the text:

But many continue to wonder just how much of The Usual Suspects, how much of Verbal Kint's (Kevin Spacey) story, is true. (True, that is, within the movie's plot.)

McQuarrie says only after finishing the film and preparing to do press interviews about it did he and Singer realize they both had completely different conceptions about the plot.

"I pulled Bryan aside the night before press began and I said, 'We need to get our stories straight because people are starting to ask what happened and what didn't,' " recalls McQuarrie. "And we got into the biggest argument we've ever had in our lives."

He continues: "One of us believed that the story was all lies, peppered with little bits of the truth. And the other one believed it was all true, peppered with tiny, little lies. ... We each thought we were making a movie that was completely different from what the other one thought."

I never consider this possibility before. It adds an entirely new level of complexity to the analysis.