I'm guessing it's all about points of failure.
What I mean is, when speaking about overclocking, the least amount of parameters/instances of things to overclock, the higher your chances of getting big results.
Getting 4 modules (or even Cores if you prefer) to overclock by 25% and being stable is harder than getting 2 modules/cores to overclock by 25%. If just a single module/core is of bad quality, then the rest can't go higher than it.
So, multiple points of failure is the only reason I'd go 2x2GB, if I didn't care about OC whatsoever, I wouldn't be afraid of going 4x1GB from a quality RAM manufacturer.
I hope that makes sense?