... legally, local (and I suspect most US jurisdictions) law requires certain reflectors on any "bicycle"... That said, no, I don't have them on any machine save my cafe cruiser...
It's more a matter of weight (yeah, I know it's small but on wheelz, it's rotational mass) and the true relative efficiency of reflectors per se.
Again, locally, you are not only required to have reflectors, but when riding after dark, both a headlight and some type of "tail" light as well, owing that reflectors are marginal at best in allowing someone else to see you at night (not only do you have to be illuminated by some other light source, whoever is looking has to be cognizant to recognize the reflection as a bicycle... or at least, something to avoid... which in my experience... is rare). Indeed, I'm sure that a Google search will reveal part of the litigious side of their use (I believe that Huffy or some such was sued because, even though the bike was equipped with reflectors, no body told the stoopid ass'd parents that that wasn't enuff for junior to go riding after dark... in traffic).
Again part deux, locally, I've never seen nor heard of it being enforced, though I suspect that as part of an accident report and/or citation, it might come to bear... but not as a matter of course.
Since most of my riding is during the day... virtually no reflectors on most of my machines (I do put a bit o 3M tape on my helmet... and one set of CXP30s have 3 blobs of 3M equally spaced on the deep section of the rim)... and those that I ride after dark, are light shows (homemade 55watt headlight and a combo of Xenon and LED tail flashers).