Regardless of their function, when wet the ones around here are lethal. At any speed, if you aren't exactly parallel to them, they cause my wheels to slip, and that's on my Tourer, Singlespeed, and Brompton. They are the only thing I've slipped on significantly in the last few years (and I cycle all year, including on snow). Going across Tooting Bec common, there are a few railings between the cycle side and the pedestrian side of the path (not that many pedestrians take any notice of the two sides). Whenever the railings start or stop, there are a set of these slabs, they are utterly incredibly unsafe to cycle over.
They may have a function for blind people, but (i) How did they ever survive before they were invented? (ii) The risk to cyclists seems far greater than the risk to a blind person.
Of course, I suspect the average blind person would get on perfectly well without them, but with our current nanny state mentality, everything possible has to be done to make the world safe for the vulnerable, even though so many of these instances are ill thought out, and on occasion significantly increase the risk to one party or another (often those who are supposedly being helped, cycle facilities anyone?)