Rrog, I have experienced the same with Altec, Electro-Voice, AR, Stevens, B&W, Celestion, Spendor, etc. Likewise, I am in an audio club and no members experience this.
There is a dope on the outer side of some cones, as mentioned by another poster, that can migrate. But really, so what? Unless one has Lowthers, which often have issues, due to the irresponsibly small voice coil gap, it's all just urban legend, IMHO.