i'll answer your question with another question... if skimmers pull out good trace elements, then how do you know which ones are being pulled out and how do you know which trace elements to add?
granted there are some tanks that can pull off a skimmerless tank, most of them have low bio loads and minimal nutrient import. i feel a skimmer on a tank adds to good husbandry and does more positive then negative on our tanks.
if you could only see the disgusting skimmate i pull out of my tank every day, you would be amazed. keep in mind that i feed very little, have no sand bed, only one fish and i have very little algae in my tank; micro and macro.
i agree with what the others say about a quality skimmer, you do get what you pay for, and 200+ for a ER or similar euro style needle wheel skimmer will do wonders for your tank. If it pulls the good stuff out, who cares... thats what water changes are for.