Bant is one of my favorite color trios, as green really appreciates the quality interaction that blue and white bring to the table
It also asks a couple of questions of deckbuilders considering it: "How much do you want a bunch of 1/1 Insects, and are you willing to play mono-green to get them?" Looking at the available data, the answer seems to be, "not all that often." That is, unless you're running an Insect kindred Commander deck with Zask, Skittering Swarmlord, Xira, the Golden Sting, or Vrestin, Menoptra Leader, in which case the answer is still just "sometimes." 14
Fuse lets you cast the left side, the right side, or both if their mana costs are paid at the same time
They buy them because they want to play the game without taking out a second mortgage or babying a deck like it is made of porcelain