I also have 'heard' this. I have them on one bike, well two, but the other is in the process of being converted to bar end, index shifting. The 'issue' isn't one I am very concerned about.

While I am certain that in a 'perfect storm' situation the shifters might cause the problem folks describe, I really doubt the problem is all that serious. All one needs to consider is that given how litigious the US, companies still sold these stem mounted shifters for over a decade. Since alternatives were available, I doubt they would have done so, if the described problem was all that common.

I suspect that this issue is comparable, and possibly a precursor, to the myth of saddle caused impotence...