Roger, the F1 through F4 keys have been used in PM as contact sheet filter shortcuts for years. F1 shows contact sheet all photos, F2 shows just the selection, F3 just the tagged and F4 just the untagged. It is an arrangement that comes natural and works extremely well for me and others
Any time you need to go back to whole contact sheet without any filters applied, you just press F1 and bingo. From my user point of view, filtering the contact sheet in the process of selecting photos, especially when working quickly under a deadline, is one of the most common things done with PM. I think it is served well by using such prominent function keys rather than a less easily used key combination.
Also, it allows consistency between Mac nad Windows versions, where historically Macs have a separate Help key anyway.
Just my two cents