I have a mid-2012 Macbook Air 11" with a 128GB SSD. On it have two installs of OS X. One of Mountain Lion (10.8) and one of Mavericks (10.9). I want to remove the Mountain Lion install so I can use the entire SSD for Mavericks (10.9). Initially, I only wanted to test Mavericks (10.9), but later decided to use it as my main OS, since I didn't notice any bugs.
I have transferred all my files over to Mavericks (10.9), so all I need to do is delete to Mountain Lion (10.8) partition and expand the Mavericks (10.9) partition to take advantage of the entire SSD.
I tried deleting the Mountain Lion (10.8) partition in Disk Utility (from Mavericks (10.9)), but it didn't let me do that. It could not deactivate the disk.
After deciding to use Mavericks (10.9) as my main OS, I realized 60GB is a little bit to small, and having another 60GB unused is simply not any clever.