I believe apps from the Windows Store have to be stored on the internal storage, but "traditional" desktop apps that have an installer should be able to be installed anywhere, might not run as well from a SD card though.
That's a good but possibly poorly worded question. I'm going to assume you mean to say install and not store. There's a big difference. Of course you can store the files that make up an app on a Micro SD card and possibly install a desktop app on an SD card. Daniielrp brings up a good point. I wonder what the difference in the data transfer rate from an Micros SD10 card to internal memory is and what it is from the solid state disk drive is. I know the solid state disk drives is enormously faster then a rotating disk. I install Truecrypt on laptops a lot where I work and it takes about 8 hours typically. I had a chance to install it on a new laptop that had solid state drives and it only took about 30 minutes.
It would be nice to have a chart that compared transfer rates of Micro SD, internal solid state disks, & high quality external 2" USB 3 drives. These are all the typical devices that we move data back and forth to using our Surfaces Pro 2's.