I didn’t say they were uncommon, I said they offer redundancy in the upgrade path. And the sample size is about 20 since you asked (not all mine, obviously)
My children went directly from Raleigh 12″ to 16″ followed by a 20″ Trek and a 24″ Kona. We had access to a 18″ wheel bike but it wasn’t used and I gave it to a friend. 26″ is really a legacy wheel size from US manufacturers, made popular by mountain bikes 😉 .
Our garage has or has had in the past: 12″, 16″, 20″, 24″, 26″, 650C, 700C and 29er sized wheels. Jumps from 18 to 20 and 14 to 16 aren’t significant. Unless there is another potential user or good resale value, I’d skip 14″ and 18″.
My 11yo has just moved from 24″ to 26″ inch, but the increase in frame size is modest (1″). Top tube length has increase significantly, however. He commuted to school this morning on a 650C wheeled road bike which is better proportioned than small-framed 700C bikes.