Upload & Sell: Off
So, I'll ask again. Worthwhile for photo-only users? Have no desire to shoot video.
Absolutely, you'll love it.
ML was originally developed for video use, but they quickly expanded their vision and efforts to exposing any and all useful capabilities that they find in Canon's cameras.
For instance, focus peaking and RAW histograms are incredible tools for still shooters. Focus peaking makes manually focusing in Live View much quicker, while RAW histograms show you the true effects of your current exposure that you cannot get with the JPEG based histogram that the camera shows you.
And there's plenty more! I'd recommend trying it out if there's a build ready for your camera. Loading it up doesn't do anything by itself, and you can peruse the options in the ML software with ease before trying any of them out.