The idea of removing skills, or the trading skill specifically, has been denied. So the thread is closed to prevent endless debates. You have your answer, so further debat on that issue is pointless.
If you need a justification for trade skill, you can think of it as your trading contacts. The better your "skill", the more and better contacts you have, so the better you can trade. After all, it's not what you know, it's who you know. If you don't maintain your skill (I.e. network) you lose it, and can't trade as well.