Actually, I think there is a simple example that shows that the same effect with the same settings can act differently on multiple tracks.
Suppose, for example, you have a given effect with identical settings on multiple tracks. Suppose, also, that there are OTHER effects on those same tracks, as well. The results of the duplicate effect can be quite different depending on the ORDER of it relative to the other effects on each track. For example, if the duplicate effect is the 1st effect on one track, the 2nd effect on another track, the 3rd effect on yet another track, etc., then the results will be different, even for identical settings of that effect. A bus with a single instance of the effect cannot achieve matching results in such a case, in general, because you can't specify arbitrary send/return points (to/from a buss) in the chain of effects on a track.
That said, the more common scenario, as mentioned already, is when you want the same effect with different settings on multiple tracks (like with an eq, compressor, etc.). Lacking the ability to add an effect to multiple tracks with an easy shortcut isn't a show-stopper, by any means, but it sure would be a nice feature! I would use it, for sure!!