The min fee on low priority txs is ~1 US cent and the dust threshold is ~0.5 US cents. What small transaction market are you talking about?
Agreed. The low priority txs is still acceptable, but if bitcoins value was say $1000 USD then a fixed threshold would represent an ever escalating minimum fee.
Summary: The minimum fee needs to be adjustable and probably the best way to accomplish this is via an open market solution - let the miners who are processing the transactions set the fee.