If your a good singer, a very simple piano accompaniment will get you very far. The chord skills you have on an arranger can pretty easily transfer to basic vocal accompaniment on the piano. Think of all those quarter note rock ballads like "Let it Be" "Imagine" "Don't Stop Believing" You really don't have to be a piano virtuoso to play that kind of stuff. I say any arranger player that sings should have at least a couple songs that they can do just as a piano/ vocal. Someday you're going to be at a party and there will be an acoustic piano, and your friends are going to ask to to play and sing something. Are you going to be a party pooper and say you can only play an arranger keyboard?

Actually awhile back I went to a nursing home looking for work. I introduce myself " Hi, I'm Paul, I play keyboards and sing...."
The activity director says " Ok, there's a piano in the corner, let's see what you got" I almost started to make an excuse, saying that I would need my special keyboard that can sound like a whole orchestra, but I just went for it and she hired me. When I did the gig, I brought my keyboard, speakers, mic..... After the show, she said, why did you bring all that equipment? She went on to say, she liked it better when I just played on the house piano and sang. So that's what I do at that place now. Less gear to lug around!