Don't be confused
The original key does not mean the "correct key"..
Original keys are the key the writer determined best for his score.. and it usually does sound the best..
The correct key is the one that accompany your voice the best..
It may not be the best key musically for the song...
I do think it is detrimental to maintain our skills...using the transpose button to play all songs in "C"... It will make you lazy...
Transpose feature is great....mostly to transpose a half step or two.. or to make vocals a bit easier..
More than that... what you play on the keys and what you hear are so unnatural ....and if you have perfect pitch, or even relative pitch...it can drive you crazy..
Just an expert's opinion.