LOL @ magpies.
Like you said, shimmer is a trend. So makeup companies have all jumped on the bandwagon.
(Glitter is a big trend too. There are entire companies dedicated to making glitter makeup). As you know, shimmer and oil provide different types of shine. Personally, I think using shimmer as a highlighter is a celeb MUA trick that the average person is getting hep to. It can give a nice glowy look. Plus, there are so many little girls buying making, to whom companies are trying to cater. Little girls love sparkle...