Whenever I install a snapshot, it's just normal. But when I install another snapshot with the other snapshot in my versions folder, the other snapshot gets deleted after I install the snapshot.
Ex:
I install 14w21b.
I do some stuff in my world.
Then I download a map only for 14w30c.
I install 14w30c.
I play the map normally as I would.
After playing, I go back to 14w21b.
Minecraft Launcher says "Ready to DOWNLOAD & play Minecraft 14w21b.
I check my versions folder, just to see that 14w21b wasn't there anymore.
If you want to switch snapshot versions, create dedicated profiles for each version you want to keep and do not use "Latest Snapshot" as version.
Otherwise older snapshots will be deleted.