Psynymph i think you answered your own question.
" maybe it all boils down to the difference between the people involved. "

You can't rate yourself with the actions of others.

What one person does with the different Doms is likely to be more fulfilling than to be with only one Dom. It's not a measure of how good they are or not but rather the needs of the submissive being met.
hrm.. as to myself .. i find that because i have too much time i am able to actively engage more than one.. ie. a wife/sub.. an online sub or two ( at times ) and i don't think noone is not receiving what they need. and If that were to happen it's time for one or the other to move on.
simple.

Cheapened is a word that can cause many problems. I think the term is misused. The placing of value on the submissive and rating her in such a manner seems to me to be wrong.