Well, my first response is NO! But I gotta hand it to you if you can use that button and play in C and hear Ab. I can NOT do that!
As I think about this issue, I am reminded of my years of playing the trumpet. I could NOT fake it at all if I did not know something. I played in jazz band, and if I had not known my scales I would not have been able to play in that band. There was no transpose button or attachment for my trumpet, but I worked hard and practiced for hours to learn to play. My band director did not allow us to get by without knowing our scales, technique, how to read music, and how to function as part of the jazz band. The drummer couldn't fake it, nor could any other instrument!
So, how fair is it that as a keyboardist because of technology one can essentially fake through a service, but no other instrument can? Is the keyboardist at least trying to learn all keys, or is this just good enough for them? If it is the latter, then it is a sad, sad state that person is in. I personally would not fake because sooner or later you will be exposed for all to see, and that is more distracting than choosing not to play because you don't know the song or the key. JMHO