An eraser tool and a paintbrush tool do essentially the same thing—overwrite an area of the image with new pixel values, typically blended in some way with the old values. It’s just that the eraser is for painting in the alpha channel, while the paintbrush also affects colour channels.

An alpha mask (on a layer with no alpha channel) is essentially a different way of viewing & editing the same data, and there you probably have no logical trouble with using a paintbrush tool.