Besides the general guideline of running dual-channel, I also based my advice on the fact that he doesn't seem likely to upgrade beyond 8GB. For most applications, it wouldn't make sense to go much beyond that without also upgrading the rest of the hardware.Depending on application, but especially in gaming, dual channel may yield very limited benefits.
On the other hand, having a single 8GB stick may make it easier to upgrade since, at least in my experience, using all four DIMM slots on a motherboard might not work (older Core 2 Duo E6400 PC refuses to boot with 2*512MB and 2*1GB DIMMs, while my current i5 750 works just fine with 2*2GB and 2*4GB).
I've somewhat got the issue you mentioned. I have 2x4GB but I would like to have 16GB but would rather not use all 4 RAM slots. However, it's very very difficult to actually need more than 8GB anyways. So far it happens from running very intense modpacks of Minecraft simultaneously with the server or else running massive fluid dynamics simulations in SolidWorks.