[ol][li] Most of the time when I rotate an image I just want to rotate what I see, not modify the file on disk. When I want both it doesn't seem like much hassle to push Ctrl-S, for ad-hoc rotations, or to use the lister + viewer pane (or thumbnails mode) where the selected file(s) can be rotated with a single-click toolbar button or hotkey of your choice.
The way Windows Photo Viewer makes permanent (possibly lossy) changes to my files just because I did something in the viewer is one of the reasons I'd never want to use it. 
(Why doesn't it also scale the image/file whenever you zoom it in the viewer? )
[/li]
[li] You can do it via AutoHotkey, if that helps.
Other than that, the options are:
[ul][li]Page Up and Page Down[/li]
[li]Space and Backspace[/li]
[li]Mouse-wheel up / down (depending on config)[/li]
[li]Left-click (depending on config)[/li][/ul]
I don't see why left/right arrow is so much easier than page up/down, space/backspace or the mousewheel.
Isn't this more about what you happen to be used to than one being objectively better/quicker/easier than the other?[/li][/ol]