Thought this was worthy of a thread of its own. I did a bit of research and found this, which shows where it came from:
I could see them having 'heavy-duty' communicators that would be better able to withstand battle conditions. Kirk took one because it was vital to contact Spock at the appropriate time, he didn't know what he was getting into, and he wanted the type of communicator that would be the more likely to survive whatever those conditions might be.

I could see them having 'heavy-duty' communicators that would be better able to withstand battle conditions. Kirk took one because it was vital to contact Spock at the appropriate time, he didn't know what he was getting into, and he wanted the type of communicator that would be the more likely to survive whatever those conditions might be.