Can you also add the second part about why this is valuable to you? Personally, I cannot see any reason for this at all, so I'd love to hear how it could help you.
It would be extremely valuable for quickly dragging guides on either the right side or bottom of a canvas that is very wide or very tall. As far as I can tell, if you need to place a guide towards the right side or bottom of a canvas/document, you can only do so by dragging it from the left side or top ruler. Additionally, it would be benefical to have measurements while working on a large document when you are on the right side or bottom without having to look on the opposite side of the screen.
I understand that while zoomed in, the right side and bottom act as scroll bars, but I never use these. I instead use the Hand Tool or the Navigator to move around, and think an option to swap the scroll bars for Rulers and Guides would be better.
If there is in fact a way to add a Guide anywhere on a canvas without having to drag from the left or top, or going through the View menu, then please let me know.