I wonder how long it will stay working, being it is below the suspension. There will be a lot of jarring of the lights.
I bought my current brake-caliper-mount Motolights in 2006, and had them on my Burgman 650 for about three years, with the stock 35-watt halogen bulbs. No problems, never changed a bulb. In 2009, I replaced the bulbs with 5-watt Cree MR16 LEDs; had those on for about another three seasons, no problems.
A little over a year ago, I sold the Burgman, and moved the Motolights to my then-new XCT, after purchasing new XC-compatible mounts and bolts from Motolights. Again, no problems.
So, you raise a good point, but for me it's a non-issue. I've never read about such problems. Do note, however, that this kind of mounting does increase unsprung weight. Given the weight of the Vic wheels and suspension components, this, too, is not an issue ... but I wouldn't put these on a MotoGP bike, for instance.