There are structures all across the nation well over 100 years old that have used rough sawn lumber as their siding. Most are still standing today. We will be hard pressed to ever get similar service out of plywood. It has its place, but it's not for exterior use. IMHO it has no business in a coop inside or out
Even marine grade pressure treated plywood will not last. 5-10 years exposed to inclement weather. The glue will begin to separate and the layers of wood will blister.
The big turn off for me is the continual 2-5 year maintenance of paint with plywood, osb, or T1-11. That money you might have saved upfront will be lost in paint.