If you change the data layout of your exposed class, you must recompile anything that uses it. That increases build times and also breaks ABI. And as the other

If you change the data layout of your exposed class, you must recompile anything that uses it. That increases build times and also breaks ABI. And as the other guy commented, the data itself has types that also need definitions and file inclusions. Without PIMPL, your data layout can change without touching your header, due to changes in a related header (even a 3rd party header).