You have it wrong. The corporate tax rate is 10.5%, if they pay themselves out of the company, they still have to pay marginal rates like everyone else on the dividends, or full rates if they take a regular salary. So theoretically, if they paid 10.5% and then took over $250,000 as employment income, they are technically paying 64% (top bracket in Ontario is 54% -but is neither here nor there). 

What they are complaining about is the fact that if they are incorporated individually, they can use the small business tax credit on the first $500K that their practice makes, now if they had 5 specialists incorporated together, they can each use the credit for their own holding company, under the new rules they would have to share it between all 5 - so only $100K of credit each. Not fair if all other small businesses get to use the same credit, and they are kind of forced into this sharing arrangement to begin with.