I regularly signal right turns with my right arm, especially if there is somebody behind me. Given the erratic manner in which most cyclists ride, drivers typically have no idea what I'm doing unless I tell them. After all, even riding in a right turn only lane is not a strong indicator that a cyclist will be turning right, given how many continue straight across the intersection from the right turn only lane.
Also, having been an avid rearview mirror user for several years now, I've gotten quite good at "reading" drivers behind me. By signaling my intent to turn right, it's clear to me that it often helps them, probably by alleviating the mystery about what the heck the cyclist in front of them is doing. It's part of being predictable.