This would be useful only if phones (Windows Phone 8, I'm looking at you!!!) allowed application data, if not the applications themselves, to be installed on the removable media. That way, not only can one make good use of the flash, but also, in case one wants to switch to a new phone, all the apps that one bought w/ the last phone can be smoothly migrated to the new one.
First, I don't know how any of this is handled in Windows Phone or if there are any hacks or workarounds. All my smartphones have been Android.
Android (at least ICS) does allow this, though in a somewhat limited form, and it wastes^H^H^H^H^H^Huses more space than storing them on the phone. Another way if you have it rooted is the Link2SD app, which does some symlink trickery to put the app on the SD card exactly as it is on the phone. None of this allows easily transferring purchased apps to a new phone though. With the official way they're encrypted, and with the Link2SD way there's no easy way to transfer the links and the stub that says it's installed.
However, moving purchased apps to a new device is already pretty easy. I associated a new device with my Google account, went to Play Store, My Apps, all. It listed all apps I had purchased for my old phone and gave me the option to install each of them on my new one.
Also, by SD card, do they nowadays mean the original SD or the MicroSD form factor? The 2 are different enough for one to easily accommodate 4 times the capacity of the other.
This is a full-size SD. Current largest MicroSD available is still 128 GB. The only difference between SD and MicroSD is physical size, which limits how much memory they can fit due to flash die sizes. The cards are the same interface as each other. MicroSD has an extra pin, but it's a second ground. Adapters exist to convert either way, but the one to put an SD in a MicroSD slot obviously sticks way out.