Couldn't agree more.
All that making these items marketable will do is create revenue for Klei (with which they will be able to introduce more content to this wonderful game), make items more easily available to the average player, and educate players on rarity and prices for each of the items (along with which items are even available in the first place!) If the price of any items are currently artificially inflated by people hoarding them/bad information on the forums about their rarity/etc, it will likely even *lower* the price of them (this happened with earbuds once they became marketable in TF2).
Additionally, there will still be plenty of traders that trade only for other DST items or keys, because they won't want to pay the 20% "Steam Market Tax" when they could get 100% of the value out of their item.
This will only increase the overall diversity and market knowledge for our virtual doll clothes