for a large purchase either the merchant or the customer may insist on an expensive escrow service that deals with disputes robustly

for small purchases this might not be so important

I saw a comment on HN recently asking why startups never challenge the VISA/MasterCard hegemony, with a response saying it is simply to expensive to enter the market with a low ROI

widespread bitcoin use would lower the barriers for this kind thing, unless you like monopolies and cartels, I can't see how anybody would argue this is a bad thing (regardless of whether you think bitcoin is good, will suceed, whaterver)