Different strains of MJ should have at least partially different medical effects depending on their exact cannabinoid ratios. The type that's good for muscle spasticity might not be as good for appetite stimulation, for example. Even so, I think you ought to be able to get the full spectrum of available effects with only a modest number of carefully chosen strains. But just because you could, doesn't mean that anyone should be forced to. How many brands of aspirin can you find on the shelf at a typical pharmacy? At least a few. So what's wrong with that?
Even given legitimate medical use, and similar effects, some users may prefer one strain over another due to taste, cost, or other factors. I don't see more choice as a "bad" thing here.
Now in terms of medical marijuana in general, I think we all know that not everyone using "medical" marijuana really has a defined medical condition that only responds to cannabis, or even that they are sick at all. Shh. . .some "medical marijuana" users are just getting stoned. There I said it.
Whether that's true or not, politically speaking, medical marijuana is sort of the camel's nose under the tent, so to speak.
If true use of medical marijuana becomes widely accepted, that demystifies the drug. If the public at large sees marijuana in widespread use by responsible functional adults who hold jobs, pay taxes, and don't abuse it, that will go a long way towards improving public perceptions of the drug, and likewise increase likelihood of more generalized decriminalization.