It depends on what the intent of the contest is. If we are intending to find the best painters on the site with the contest then yes the pros should be allowed. If we are trying to encourage members to participate in a friendly competition that would be completely blown out of the water by the pros then maybe not. Perhaps we could have a tiered competition with Amateur and Advanced levels or something?
Full disclosure: I have won two Golden Demons and a Silver and was a full-time professional painter for about 5 years. I have yet to enter the comps here because of time constraints. With more high-level entries I may pit myself against them though.
Strange, but not a Stranger.