The common one you get from people is either they get into the water and damager wild life, or they don't bio-degrade.
If its damage, then if you take care to dispose, how is it an issue?
if they're not biodegradable, then how do you dispose of the millions of bags that are thrown in the trash every day? where do you put them?
If its bio-degrade, I dont get that either. They arent the largest things around. Is it a significant issue?
you under-estimate the number of plastic bags thrown away each day. They aren't only used in supermarkets for your groceries. Practically every store uses them (clothing, electronics, books, everything). There is also plastic packaging. Plastic bags ARE a HUGE problem.
"A complex system that works is invariably found to have evolved from a simple system that worked." -- John Gall, _Systemantics_