Increasing the number of "running" apps held in memory isn't the only benefit of more RAM (for Android - I can't speak for other mobile OSs). On devices with more RAM, Android may allow an app extra space, which the app can use for more caching, etc. This page has some details -
Managing Your App's Memory | Android Developers . So an app written to take advantage of this may run faster on a device with more RAM than an equivalent device with less. There may also be cases where the garbage collector can be less aggressive, which may also help. This is a complex area.
On a device with more RAM, WP can also afford apps more memory. No difference.
However, while you are theoretically correct that apps could use some of that extra space for more caching (assuming that even makes sense for an app, which for most it just wouldn't), the real question is not what apps
could do, but what they
actually do. I'm not aware of a single app that uses memory in that way. Most senior developers would frown on such an approach, because software that behaves differently based on how much memory it can allocate, which on Android can change between sessions, is notoriously hard to test. Particularly on Android you'd also risk causing your users endless frustration, as your app will sometimes perform better, based on one set of caching rules, while another time it will perform worse, based on another set of caching rules. Most users will think your app is just out of control and behaving arbitrarily.
What do apps then use that extra memory for? Typically it's more along the lines of higher resolution textures in games, or buffering a larger segment of an audio/video stream. Stuff like that. Not dynamic caching schemes.
So, while you are correct in theory, it doesn't make a difference in practice.
Anyway, I'll let you have the last word. It's an interesting topic, but this isn't the right thread.