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You may have a point IF the original MSP had been developed for Windows. Then the addition of the Android app would have needed the development of a tool or some other way to allow synchronization between the 2 OS'es. 

With the addition of an IOS version, the ability to interact between them will increase in importance. 

Since it did not happen that way, the Android version was 1st [MobileSheets, then MobileSheets Pro]. I think the folks that used it [or Mike] wanted a way to work on material on Windows then transfer it to Android, so along came the Companion [MobileSheets]. Then the Windows version was developed and an IOS version was planned. Since there was a tool already available to communicate between OS versions, why reinvent the wheel and complicate things?

I'm sure Mike could devise code for each version to add interaction between OS versions, but it would add to his maintenance workload. The Companion is a 'lowest common denominator' or 'common module' between the OS'es and is free. I'm assuming, of course, that a Companion tool will remain the common communication app between OS'es.

Just a note, You will not need the Companion with the Windows version. You will actually have what you expect.