As it is, Excel needs to keep around a) the value of a cell, b) its format (ignoring formulae, comments, etc.).

In your proposal, Excel would need to keep around a) the value of a cell, b) its format, and c) whatever you originally typed in, with c) being potentially more than a)+b), without very much benefit. And remember, Excel was released in 1987 (years before Windows 3.1) on machines that had less storage than they have today. I'd say the design decision made back then was the right one.