The only problem with adding copays to Medicaid is that they would mostly still utilize the system, but just not pay their copay - "i forgot my wallet", "I will bring back a check", I can't pay today etc. I see this all the time in the office and mostly just get burned on this - we have a basic health plan that pays Medicaid rates plus 15 % or so with a $15.00 copay, some number of these pt. always have an excuse and unless I get really strict and don't let them come in without a copay - they abuse us and don't pay their bill.
Adding a copay just means more write offs on our bad debt - really doesn't help the system.