Knitting a sweater sideways is an excellent way to get vertical stripes without doing any stranding. I’ve also found through wearing the sweater that sideways knitted fabric doesn’t wrinkle as readily as vertically-knitted fabric, so at the end of the day the body of the sweater is much less rumpled than the regularly-knit sleeves.
Unfortunately, there are also downsides to sideways knitting that I failed to account for when I was planning the sweater. See Exhibit A:
For a set-in sleeve, the sleeve seam should lie directly on the edge of the shoulder: the white line in the photo. And originally, the sweater did sit there. Over time, however, the sideways-knit body stretched horizontally, so my sweater tends to look more like a hybrid of drop-sleeves and set-in sleeves. The next time I knit a body horizontally, I will either use cables to reduce the stretchability of the fabric or make the whole sweater a bit narrower than I typically do, since I now know it will stretch with wearing. Estimating from the picture, I think I gain about half an inch in width on each side (my row gauge here was 7.7 sts per inch), so in the future I’d substract an inch from the width of this kind of sweater.
Just a note, this may not be a problem that occurs with other yarns. The Jaeger Extra Fine Merino DK is unusually drapey for a wool yarn, which makes it gorgeous and comfortable but also more likely to stretch out over time. The stretching problem would probably occur for any yarn, and be particularly extreme in dense fibers like cotton.