There are many reasons why Mass Storage Devices are not always a great solution:

First of all, MSDs are expensive, for several reasons:

• That FAT file system takes lots of code space, often requiring a bigger, more expensive processor.
• The SD Card Association wants a license fee of about $1000 up front, before you even start developing your device.
• When you're finished developing your device, and are ready to sell it, Microsoft wants a royalty for the privilege of using their patented FAT file system.