If users want to pay with a huge transaction then it seems to me the user should cover that cost. Allowing users to pay merchants with 100K transactions full of dust and expecting them to eat the cost seems like a great way to enable bleed-the-merchant-dry attacks.

RE: hiding or showing fees:  I pointed out to Peter that there doesn't have to be One True Answer.  Let wallets experiment with either hiding or exposing fees, and may the best user experience win.