Well, most libraries behave very badly when symlinked out of rootfs. So when you apt-get libboost1.42-all-dev you will be left with few mb out of your current 50 (yeah, full libboost is a cow). Then you start following all optifying instructions out there, get back to around 40mb free and another cow-dev comes around, you end up <10mb, optify some more... neverending story really.

EDIT: about speed: copy 10mb file for a test to your ultrafast NAND, you will not end up with 40mb left, more like 43-45, depending on how compressed it is. I assume that it is compression on the go for rootfs (if someone has better explanation, lets hear it), which makes microB binary for example start much faster from slower eMMC as it then is uncompressed and starts almost instantly. From compressed NAND it uses more CPU and takes longer to start (at least my experience).