When I buy Island as an opener, I'm not happy if I draw Island-Copper-Copper-Copper-Copper, but if I do, I will use it on the Copper, absolutely. I think I would do this even if I also had, or planned on using, a trasher -- even a Treasure-only trasher, like Moneylender -- for the simple reason that if I don't get rid of the Island at the earliest opportunity, I've added a dead card to my deck instead of getting rid of one. The longer that Island sticks around in your deck, the less benefit you get when you finally do use it. Because eliminating a card from your deck is a benefit that compounds with every reshuffle, while having a dead card is a penalty that compounds likewise.
When I get an Island in the mid/late game, I still might use it on Copper but I'm a lot less likely to. The further you are into a game, the less deck-thinning helps you and the less dead cards hurt you. So in that case it depends on the situation. Certainly the presence of Moneylender or Spice Merchant in my deck (or cards that like Treasure, like Stables) will reduce the likelihood that I'll use Island on Copper. (For that matter, Moneylenders and Spice Merchants that have done their job and outlived their usefulness are themselves good Island targets in the mid-to-late game.)
