Disk Utility's ability to merge partitions is very hit-or-miss. Sometimes it refuses to do it at all, sometimes it threatens to erase the entire device and all volumes on it, sometimes it threatens to erase one of the volumes, and frequently it claims it can do it and when you try it eventually fails with weird errors. (or stalls / hangs in the attempt)
Disk Utility has become one of the WORST pieces of software inside OS X, for many reasons. I'm often left to dig around in terminal and do what I need to by running Disk Utility from terminal. There, it either works as expected, or fails with an actual useful error message, or at least an error message that is helpful enough to explain why it's failing, what I need to change, or give me a clue as to how I can rethink my process to accomplish my goal. So it's not diskutil, it's Disk Utility (the GUI front-end for diskutil) that I have grown to despise.
