Multi-Doctor Stories are events when the BBC takes advantage of the fact that the Doctor gets played by multiple actors. Suddenly, we have aging, unhappy actors traipsing around the stage being unhappy at the changing quality of the program, crowding the current Doctor out of his own series, and generally illustrating why they're not around anymore. Furthermore, the effects this should have on the canon of the prior doctor are never really addressed. Multi-Doctor Stories are Timey-Wimey, and violate the Blinovitch Limitation Effect.
Multi-Doctor Stories are great, in other words.