And there could be a backdoor that could potentially wipe all your saves.
Sure it could get patched, that doesn't help people already affected.
A backdoor implies it was put there purposefully. Are you saying that Badspot cares about peoples save files enough to put a backdoor to be able to delete them? Because I'd seriously bring that into question.
Or even more generally, a backdoor of any kind. He's trying to run a business, if it were revealed that the game had a backdoor that allowed deletion of files, he would lose TONS of future sales (Imagine every review on steam calling him out as being a malicious douchebag), not just on Blockland but any future games he makes as well. It wouldn't be in his best interest, and Badspot isn't stupid.
But to address that in a more general sense, which you seem to be saying is "Until they get patched, having it open-source doesn't help the people affected."
Same goes for closed-source though. Until say, corner jumping or that walking-through-walls glitch, the people affected by it aren't helped by it being closed source.
I think, also, in this specific scenario, that having the system open-source wouldn't really increase the number of exploits. You see, Blockland is built on the Torque Game Engine, an engine whose code is available online if you're willing to do some looking. Many of the coders here have it already. Anyone who has experience in reverse engineering and finding exploits have probably already found the source code or just reverse engineered the EXE file, which is all you need to do to find exploits in the first place. If they haven't found it before, having the somewhat modified source probably won't help them much if at all.