Regulating interchange fees would eliminate a powerful incentive for payment schemes to create innovative products and services which benefit merchants. Payment schemes should be able to compete with each other to make their products more useful and valuable to merchants, their incentive being the ability to generate higher interchange and, in consequence, the ability to attract banks keen on receiving more interchange.

See the full blog post at:

http://aneace.blogspot.com/2007/10/why-unregulated-interchange-model-is.html