yes, the mkfs program generally reserves space for non-root
it's actually a good thing to reserve the space, its prevent fragmentation when the disk gets quite full. The 10% is a historic thing from when disks were generally less than 1GB! If you've got a multi-GB disk then probably a few percent is OK (e.g. reserve perhaps ten times the size of the most common file?)
in flash memory, it could prevent premature wearing out because when files are rewritten they can be written to a different set of blocks; if the disk is 99.9% full then re-writing a file will inevitably have to keep rewriting the same blocks.
I don't know if the flash controllers on the cards understand the fat filesystem enough to do any special wear levelling, but that might be the case and explain why cards seem to wear out quicker using ext3. I would propose that it's a good thing if you do put a swap file on the card to regularly delete it and create a fresh one so as to spread the wear.
--edit
