Income sprinkling and multiple capital gains exemptions are red herrings. The taxation of passive income is the real turkey.  It is devastating to those who hope to retire, look after themselves during a disability or fund maternity leave. 

Keep in mind -- doctors very seldom sell their practices for any significant amount of cash. With too few doctors and too many patients, new doctors are fully booked very soon after opening a new practice. There is seldom a need to buy out a doctor in such circumstances, so buying a practice is very affordable. 

As for dentists, a good dentist might sell his practice for one times gross income, say in the range of $500k or less. There is no multiple capital gains exemption as the capital gain doesn't warrant it.

Exempting the sales of small businesses and farms (as opposed to professional practices) from tax encourages them to be passed down to their children. Otherwise, these companies in times past were often sold to non-residents.