I don't think that this method is necessarily for those wanting to use multiple cards at once, but to allow those with multiple cards to swap them wherever they may be. I sure don't want to be carrying these tiny things in my pocket, so this is great for me.
The included card may have a high capacity, but it won't be that hard for some people to fill up. I have a 32GB iPod Touch 2G and I fill that up pretty easily once I've only added my music collection.
I have a 4GB and 8GB micro SD card. Add the 16GB card that comes with this phone on top of those, as well as the 8GB internal memory, and I'll have plenty of storage to load up as much content as I please. If it weren't for my iPod, I would actually probably end up pushing this limit.
I figure I'll load up whatever emulator ROMs I want at one time onto the 8GB card, all my movies and videos can go onto the 16GB card as well as with any music that I decide to put on there for whatever random reason, and all my pictures can go on the 4GB card. The 8GB internal memory is, of course, for application data.
At first I'll probably just leave the 16GB card in there all the time until things start getting cramped, then I'll begin to move content onto other cards. No real need to make things complicated right off the bat when it's just unnecessary.