I have PS Touch on my Android phone and it works well for what it does, given the size of the screen and all... but to me LR is so much more - file management, keywording, processing, printing, etc... Since the current recommendation for LR5 (not the official 'minimum' requirement) is 8-16GB of RAM or more, I can't see being able to do much with raw images on a tablet. Also, even my 15.6" laptop screen is barely adequate for me. It might be useful for DAM or pushing around cell phone images, but since (I'm assuming) most people would use it for traveling, slow wifi at hotels, cafés, etc. would make it a challenge. Time will tell.
And no I won't invest in a subscription, but that's a personal preference.
Mike.