I think that once again many are confusing arranger playing with piano playing. They are two different instruments and require different skill sets, even though of course the arrangement of the keys are the same.
But that's another feature of arrangers, you CAN play them like a piano, to a degree.
Having never really played piano, I think more like a guitar player or other instrumentalist.
I don't think good musicianship can be determined solely by whether you play good piano.
This of course even relates to what we regard as "good feel" or "a great key bed". Having fully weighted keys can be a deterrent to good arranger playing, while being somewhat essential to good piano playing.
I somewhat envy those who can play good left hand bass while singing and playing with the right,because it opens up avenues for using the arranger that I don't have. I can play left hand bass, even bass guitar, but it contradicts the way I basically play arrangers. I haven't developed that particular skill set to a degree where it would enhance anything, and at this point I don't have a desire to do that.
The one reason I keep looking to replace the excellent Korg PA4X is because the semi-weighted keys hurt my fingers after a few hours. So far nothing else has filled the bill completely.
As far as singing, I started trying to do that when the singer quit the band and I wanted to keep getting paid. I was AWFUL and it took years to become adequate. I'm only in the past few years starting to think of myself as a decent singer, with many limitations. Maybe 50 years of doing it every night helps one learn. I'm still learning things...such as how to determine which keys sound best for me, experimenting with playing in different keys. The transpose button works in many cases, but the "licks" you play in one key can be much more difficult to achieve in another key. Actually playing in other keys often sounds different than just transposing everything. But, again, that's another great feature of arrangers; you don't have to play in keys that are difficult for you if you don't choose to invest the time and effort to do that. I can play in any key, but am much more comfortable in "white" keys, such as C,D,E,F,G,A and even Bb. Many good players are more comfortable in Eb, F#, B, but I only use them when I have to. That might be required on the rare occasions when I use music or a lead line.
Sorry for all this rambling, and it's only meant to convey thoughts that relate to the way I use arrangers, in case anyone is interested. It isn't necessarily the way anyone else should. There is no right or wrong way, as long as it brings pleasure to the player and/or the audience, if any.