For many things, that's only true as a technicality. Having access to the source code of a complicated piece of software doesn't instantly make it easy, or even remotely feasible, to fix it yourself.
For example, back when I ran a Linux desktop and had problems with my video card or video settings, there wasn't a chance that I could fix it myself. The odds of me being able to do so were roughly equivalent to the odds of being able to fix a broken closed-source video driver by opening the binary up in a hex editor. Technically possible, yes, but not remotely feasible.
How so? You could similarly beg or pay for Apple to fix the issue.
Consider OP's use case. Lets say hes a non-programmer and Keynote is open source, but similar in complexity to WebKit. Now he comes across the error - what can he do? Learning to program or finding someone with the time and knowhow of Keynotes (or WebKits) inner workings may take months.
On the time scale of months, he could also bitch enough at Apple that they may release a tool.
However in both cases, the most time efficient solution is to download Keynote 09.
You can also patch the binaries of proprietary software, technically possible, but exactly as unfeasible as your solution when you want to get stuff done.
For example, back when I ran a Linux desktop and had problems with my video card or video settings, there wasn't a chance that I could fix it myself. The odds of me being able to do so were roughly equivalent to the odds of being able to fix a broken closed-source video driver by opening the binary up in a hex editor. Technically possible, yes, but not remotely feasible.