Mostly for noise margin and speed of the edge reasons. The noise margin is already pretty low since the high and low levels are near the margins (and much worse than a typical chip output's
margins). As well if the input transition is slow the schmitt trigger input will clean it up. If the
2 resistor trick works (which I doubt, the high level is I think too low to trip the input of a 328 from the data sheet., although I haven't tried it) more the power to him. I tend to prefer solid designs though.
Peter Van Epp