Hi F11#13 (is that right?)

If your melody has a "g", how is the arranger supposed to know you want it as the fifth of a Cmaj instead of the root of a G (etc.)? This is the problem. Specifying a root solves this problem to some extent. However, to distinguish a Cmaj from a Cmin would require that the third be specified. Still, this is something feet can accomplish while both hands are tied up with a guitar or wind instrument. A sustain pedal is not required assuming there is a chord hold function.

In principle, you could specify a scale instead of a chord. There would be fewer changes. Vocal harmonizer have a scale mode, but I do not know of any arranger keyboards that do.

Clif