I would not use the transposer if I was playing two handed style piano parts. Certain voicing aren't going to sound right- too muddy or too high.

On an arranger, that's so much of an issue. So yes I do use the transposer when backing a singer that sings in a diffenent key from the original. I alwyas mean to get around to learning the tune in the key of the singer but that doesn't always happen.

I do enjoying playing in all keys as an exercise. The other day I was practicing the bridge of "One Note Samba" in all keys. Now I can "quote" that melody in other tunes that aren't in the same key.

I like to take a blues lick in the RH and a boggie patern in the LH and run through all the keys.It gets especially tricky in C#, B, Ab, F#. I can do it in the practice room but I'd hate to have to play blues in C# on the band stand! It is possible but I'll leave that to the professionals. I have a day job, a family.... Rather than learn tunes in all 12 keys, I'd rather spend my time learning new tunes.