Though hard disk is cheap these days, ssd is still expensive. My Android SDK is taking something like 14Go.
I have installed apis 19,20 and 21, and each is taking around some valuable space (21 uses 9Go!) in the system-images/ folder.
Is there some cases where I do need 19 and 20 if I already have 21 ?
You will need 19 and 20 only if you plan on creating projects targeted (compiled) for those versions. And you most probably do not ;-) Also, avoid downloading everything each release offers if you do not have a specific need for all of them.