Why are internal pixel types no longer deduced from ElastixRegistrationMethod<TFixedImage, TMovingImage>?

[N-Dekker]

Commit faf20af (@kaspermarstal, September 4, 2016) says:

ENH: Allow ElastixFilter and TransformixFilter users to manually specify internal pixel types. Previously these was automatically deduced from instantiated template parameters

Does anybody recall why the elastix "FixedInternalImagePixelType" and "MovingInternalImagePixelType" parameters should not be deduced from the ElastixFilter<TFixedImage, TMovingImage> template parameters?

(Note that in the mean time, ElastixFilter is renamed to ElastixRegistrationMethod<TFixedImage, TMovingImage>, and it underwent quite some changes, but these internal pixel types are still not deduced.)