It would be very expensive to perform math operations on a number stored that way, it at all possible.

Assuming it's possible, even straight up comparisons would be super expensive. If you multiply m = 3 e = -1 r = 1 by three, you get m = 9 e = -1 r = 1. But that's equal to m = 1 e = 0 r = 0. Should every math operation perform this expensive normalization? Should comparison? Neither are appealing.

And to what end? To avoid rounding? You'll need to do that anyway when r != 0, and you usually want to do that regardless. To make int work better? int will still give the wrong result in the example above (m = 9 e = -1 r = 1) without normalization.