I've never seen profit sharing work as a motivator; in most companies it's too decoupled from individual performance. You can work yourself to the bone and still receive a pittance if the company is doing poorly for reasons beyond your control as an employee.
Where 'profit sharing' works, it tends to be called something else: a commission, for example. Works pretty well with sales staff. Equity is also a good motivator if there's a reasonable chance it will be worth something. What's important is being able to connect individual effort with reward. Otherwise, it's not much better than a lottery.
Since we are both obviously just talking about our own experiences, I can state quite definitely that knowing my team was gets 25% of the REVENUE from each of our projects is a great motivator. Not to work insane hours, but to do our best, knowing that we were being treated very fairly for our efforts.
If the success of the company is not influenced at all by your efforts, then that must be pretty depressing. Maybe a good sign that you would be happier somewhere else...
Where 'profit sharing' works, it tends to be called something else: a commission, for example. Works pretty well with sales staff. Equity is also a good motivator if there's a reasonable chance it will be worth something. What's important is being able to connect individual effort with reward. Otherwise, it's not much better than a lottery.