The major point I see against licensing cyclist goes like this:
Cars are licensed and insured because they're incredibly dangerous. Licensing reduces the public safety risk cars pose by ostensibly maintaining a minumum level of skill in traffic. Liability insurance covers drivers for the damage they can do to public and others' private property-- which can be astoundingly high. When was the last time a bike lost control, rammed into a house, and killed a family of four?
The truth is, the stakes are just higher for cars, and that's why they're licensed and insured. If we want to address the problem of illegal or reckless cyclist behavior, we've got to realize that it's not an issue of licensing and insurance-- it's an issue of law enforcement and education for cyclists and drivers alike.