Should you be promoted if you have never performed your MOS function?

I would say yes. But, that Soldier needs to understand it is their responsibility to rise to the occasion and ensure they perform their new duties to standard.

SPC Robert Bergstrom,
I am of the opinion that you should not even be considered against those that have been doing their MOS all along, but that's just my opinion. We all know that exceptions get made all the time. If you are in a special assignment, for example, should you be penalized? No, I do not believe that is fair to the Soldier or the branch, but it definitely depends on the circumstances. I would not agree with every situation as I'm sure you wouldn't either.

I don't know about the AC, but many RC senior leaders are placed on their LEADERSHIP abilities as NCOs (and officers). I can't remember the last time my CSM was in his or her MOS.

Low density MOS's and overstrength MOS's make doing that particular skill more difficult. Often in the support fields, you find yourself being a leader/mentor for several MOS soldiers and do your own less and less.