At least with common themes, toggle buttons look exactly like command buttons. One might say that label and context make clear what is what, but I think things that behave different should look different.
Toggle buttons lead me to check buttons, as they express the same thing. Plus an inconsistent state, best known from cases where check buttons are used to indicate and change properties of selected items. Radio buttons are like check buttons, but are for mutually exclusive options.
Time and time again I catch myself aiming at the rectangles or circles, even though I very well know that the labels belong to the target areas, too. So how could check/toggle and radio buttons with obvious target areas look like?
One could modify the shape of toggle buttons, but the meaning of that kind of hint would have to be learned. It would also limit theming possibilities.
With both radio and check buttons, the iconic part could be put on buttons. Pretty clear, somewhat chunky and not too space efficient. Next: taking inspiration from some audio gear and truly mixed inconsistent states 🙂
Taking a check boxes right out of the GIMP to see how the same line spacing works combined with buttons. Plus a “flattened” style to be less loud.
Trying to make the mutually exclusive nature more clear.