Properly incentivizing staff can be a powerful motivator to improve performance, however care must be taken to devise incentives that will elicit the specific performance outcome you desire, while fitting within the margins of your business.
A percentage of profit may very well be a good incentive, for the right individual with the right responsibilities. An across the board percentage of profit for all staff is likely a bad idea however.
Profit is bottom line, and in this scenario you are essentially tying your staff to all revenue streams less business expenses. This could make sense in the case of an operations manager or other individual who is intimately involved with both managing expenses and overall growth and success of your business. However, a regular staff member whose singular responsibility is coaching great classes is likely not the best candidate to incentivize with a percentage of profit.