Totally agree - if your hands are on the keyboard, then use the keyboard shortcuts, no movement required.
If your hand is on your mouse, though, this means you don't have to move that hand back. (I'm assuming left-hand mouse uses won't find anything involving TAB to be hand-agnostic. : )
My usage pattern tends to be keyboard - search, tune search - then mouse - open lots of new tabs, then scroll up / down with mouse, and flick between those new tabs with the mouse, perhaps select some text with the mouse.
I think everyone agrees that frequent relocation between mouse / keyboard is sub-optimal.
These hotkeys are functionally the same as Ctrl+Tab and Ctrl+Shift+Tab, which - at least in my idle position - don't require moving your hands any meaningful amount.