For example, a Mixer node will have three inputs. Two audio channels and one channel that provides the mixing level, so the inputs are not all homogeneous, and it matters whether the slider is connected to an audio channel or a mixing level (because one is a type error and the other is not).

It's just occurred to me however, that this is the Input class. Which means rather than the question being "why are we limiting the number of inputs", the question is now, "when will the number of wires incoming to one input ever be more than one?" (never), This logic is slightly unnecessary, you should just be making sure the input isn't already occupied by something.