When you perform on a keyboard I believe the first sound the audience expects is either a piano (acoustic or electric) or an organ. Those are just the most natural tones for a keyboard to make. And unless you rely 100% on organ sounds, sooner or later you will probably be expected to play a piano piece so your piano sound should be at least as good in quality as the other sounds in the keyboard.

The acoustic piano is one of the most difficult sounds to emulate digitally and so it has become a benchmark, and not just by users but by manufacturers who tout their keyboard's piano sounds. This is true for pretty much all rompler workstations, not just arrangers.

I am a pianist and if I own a 61 or 76 note keyboard then I also have an 88-note graded hammer action MIDI controller to go with it. It would be a strike against any keyboard to have pianos that don't sound good, so the other sounds or features would have to be exceptional for me to buy it since I would still need a good piano emulation to go with it. There was a time when I used a Roland EM-2000 arranger with a Kurzweil PC88 as a piano and controller - the EM-2000's piano were not good enough for my ear but the other sounds and MIDI playback/accompaniment features were exactly what I needed.