I have never seen a 1.5lb plate.
Most bber's shoot for golden numbers, like 315, 365, 385, 405, 455 because of the evenness of the plates on the bar. When you hit a 3 plate bench, I think the risk of possible injury in going ahead to try the same bench only with a couple of dinky 2.5lbers added to the side is not worth it. Especially when you start hitting 400+.
Reminds me of my old gym where there was this brother who was a trainer. He was probably about 5'8" 200lbs. He would always be training this one Italian guy who was not nearly as strong as he was, but the trainer would always find a way to work in his own extreme lifts for "educational purposes" I guess. Basically it was showing off, but I won't hate because he's got a very respectable physique and strength level. Anyways one time he was training the Italian dude on legs and decided it was time to show him what proper form on a 500lb squat looks like. Not to mention this Italian guy had probably a 285 parallel squat and would probably never squat 500lbs in his life. But I just watched the trainer load on the two dinky 2.5lbers on the side after loaded up 5 plates. It just struck me as trivial to have the dinkies on there. Anyways he squatted it(probably about 4 inches above parallel). The end.