Why isn't there a separate field for a Middle Name. If a middle name is included in the Name field you cannot do a logical search and get a hit on the individuals name without knowing a middle name has been included/omitted.
It is only in the modern U.S. that middle names are considered to be somehow a different species than the other parts of a person's name. In the rest of the world, people may or may not be given more than one name, and the names are all treated the same. (In most places, you're free to choose which parts of your name you use or don't use, except on very official documents, where you're usually expected to use them all.)
FamilySearch's search algorithms will all match any name in the corresponding name field. For example, if I search for Michael John, it matches Michael John and John Michael, of course, but also people entered as just Michael, or as John, or Michael Lewis, or John Stephen, or even Johannes Nicolaus. Searching for just John will also match Michael John and John Stephen, and all of these searches will also match people with just that initial (ranked lower than the full names, but still there). Therefore, there is absolutely no reason for a separate middle name field.