Rebreakables are cheaper (in the long term), more environmentally supportable, and more consistent break to break than wood. If breaking is a part of a test, there's no reason not to use them.
I just remembered a funny story related to your post...
My former sensei opened a new dojo (relocated, not another one), and we had a Grand Opening day. We were breaking boards left and right, beating up a guy dressed up as a dojo clown, etc. I know it sounds McDojo-esque, but we were just having fun and not taking ourselves too seriously.
The head of our organization came to show his support. One of the guys was trying everything he could to break this single line board. He punched the thing about 8 times, and he was putting everything he had into it. I couldn't tell what was going wrong from my angle. He broke a ton of boards earlier, so it wasn't like he couldn't break a board. He brings the board over to the head of our organization and asks "what am I doing wrong?" The head guy laughs and says "See all those knots? You probably couldn't break that thing with a sledgehammer!"
There were about 8 knots right in the middle of it, and about 10 knuckle indentations in it. One of the funniest things I've ever seen.
One more that I was told about in my current dojo...
There was a black belt who'd won the breaking championship a few years in a row in our annual organization tournament. One year, he failed at a single concrete block (he'd broken several at a time every other year). After his DQ, he set it up off to the side and was determined to break it. He hit the thing several times. Another guy comes up with a sledgehammer and says "here, at least hit it with this." A chunck flew off and revealed there were iron rods in it (rebar?). He bought it himself, so it wasn't an intentional setup. The store he bought it at mislabeled it.
Everyone who knows him now buys an extra one to break with a sledgehammer before attempting to actually hit one for quality control purposes.