Thats actually a different approach than how I had been doing it - i had been loading a hidden spritesheet by grabbing it from the dom (this is all just prototypal, playing around with canvas code, nothing "production" quality mind you), then keeping track of the position in memory and iterating through the frames using the overload of drawImage that lets you specify a slice of the image as what you want to draw.
I wasn't even aware of using background-image-position as a way to do this... I'll stick that one in my cap, thanks!
I'd say the main benefit of spritesheets as i've seen it would be what you've already commented on: Fewer GETs against the server (when you're thinking in terms of scale, for a large project with lots of assets, i'd say this becomes important, but of course that's a fairly subjective metric), as well as organization of files.
You could go with naming conventions... but then for a 45 frame animation, you have 45 distinct files all named something like "SPECIFIC_ANIMATION_FRAME_X", as opposed to using a dynamic approach, utilizing the cell-size of the frames and the width and height of the image overall, to calculate the frames on the fly; To me, that is far more maintainable than a huge pile of aptly named images.
You could make the claim that calculating out the frame each time is wasted cpu cycles that could be spent elsewhere, but I feel like it's the right approach.
But that could just be my opinion. I would love for someone to weigh in with actual metrics on whether or not using a dynamic approach is actually faster or slower in a meaningful way than a brute-force style naming convention animation system.