The raison d'etre for shoetrees is to prevent your shoes, after wear, turning heavenward at both ends and looking like a piece of dried-up toast. They are shoe straighteners not shoe stretchers (different animal altogether). Once your shoes have dried and straightened out, after let's say 48 hours, there is no necessity to keep them on shoetrees until they have been worn again. Although it's nice to have, you don't need to have a pair of trees for every pair of shoes. There is one bone of contention between European and American makers of shoetrees. European trees are made of hardwood (usually beech) and lacquered; American trees are made of softwood (usually cedar) and left raw (Although in the past they were made from lacquered maple). I have seen it claimed that raw cedar, if left in for long periods, has a tendency to dry out the leather; i.e. they are very good at absorbing the foot moisture but then they go on absorbing and absorbing. This problem apparently does not occur with hardwood shoetrees. I don't know if this is true or just mud slung from one manufacturer to his competitors. Most of my shoetrees are hardwood (Chinese, they were the cheapest) and the few cedar trees I have I keep for shell cordovan shoes. (They've gotten so much fat in the tanning process; I believe you can't get them to dry out.) Alternatively, if you keep your shoes on cedar over long periods, give them a good coating with leather balm (leather food) from time to time.