Would you call Yanni a pianist? Would you call the guy who plays the keyboard for Evanescene a pianist?
Uhmm, I don't know
There a lot of famous composer/pianists that never took formal lessons but started playing them themselves when they were young
Now, I would call them pianist
Now, I swim several days at week
It my sport, not my profession
Though, even if I'm not a professional swimmer people call me a swimmer
So the question should be
Are you a pianist even though you're not a professional pianist?
The question depends on the skill and the amount of time devoted to piano
Anyway, if piano is your profession your skill doesn't count
So, someone doing pianobar, playing the piano for a group or doing just three chord to accompany a singer is anyway a pianist and that's how is called
Bottom line: since in the vocabolary there's no a different word for professional virtuoso with diploma pianist and not so advanced pianist we should all be more specific using the terms:
"prefessional virtuoso pianist"
"professional less advanced pianist"
"professional virtuoso piano concertist"
"amateur virtuoso pianist"
"amateur less advanced pianist"
"dedicated piano student"
"more sporadically dedicated piano student"
"dedicated piano plunker"
"more sporadically dedicated piano plunker"
Now, who should I contact to have these added in the Merriam?

Daniel