Performance assessment and drinking Kool-Aid

This last week, I provided a performance assessment to a subordinate.  Though I understand the necessity, I'm not crazy about doing them.  Regardless of how objective the scoring criteria is, there always seem to be someone who sees the cloud in the silver lining.  The first question I get asked is, "why do we have to do this"?  Let me break out my trusty PMBoK, as if I need the excuse. Section (page 237) of the PMBoK, it states

the project management team makes ongoing formal or informal assessments of the project team's performance.  By continually assessing the team's performance, actions can be taken to resolve issues, modify communication, address conflict, and improve team interaction.

Though I try to be fair and balanced, I understand I sometimes must make uncomfortable and unpopular decisions.  When I completed my scoring, the results were mixed.  In some areas, this person exceeded my expectations.  In others, they fell short.  It was interesting to see the incited response.  "Why didn't I get a perfect 10!!?"  I calmly responded, because nobody is a perfect 10.  That's kind of a half-truth.  I do believe in outliers.  But, this person is no outlier.

I went over to the white board and drew a bell curve.  I then tried to explain that my scoring put her roughly in the middle.  There were areas which needed improvement and those were the facts.  Why do some people feel entitled to getting credit when credit is not due?  A percentage of people will exceed and a percentage will fail.  It's simple probability distribution.

What I didn't understand was she completely ignored the good rating and focused on the bad.  When push comes to shove, I'm the one doing the assessment.  I do believe I should explain myself.  But after that, people need to focus on themselves.  When I get assessed, I expect honest feedback, so I can do a better job.  There is always room for improvement.  Giving me 10 out of 10 across the board may make me feel good momentarily, but then what?

Don't think I'm cold and calculated when it comes to dealing with people.  I would love to give everyone good scores, but then what would that say about me?  You'd say I've been drinking some strange Kool-Aid.  When people are doing a good job, I tell them.  If people are doing a poor job, I tell them.  If you don't want an honest answer, don't ask the question.

Does anyone out there have a recommendation for an objective (versus subjective) performance scoring?  What about ideas to motivate those who do not motivate easily?