With a few plugins and extensions you can really do amazing things with this type of setup. The problem is that your spreadsheets basically become an unmaintainable and impenetrable nightmare and all your business logic is held hostage in various people's personal sheets. Making wide changes becomes hard/impossible and doing something that in a traditional software framework would be easy ("Let's change the frobnicator to twizzle first and then twozzle afterwards, instead of the other way round as it does it now") become very difficult requiring many many changes in many many sheets with a lot of risk, and a lot of dilligence to be sure you have actually made a particular critical fix everywhere.