Yeah, I never understood why they didn’t put an EXT slot on the Xbox 360 (or the PS3 for that matter). Especially the PS3 is a bitch with memory resources as you actually only have about 216MB for the CPU (256MB-50MB for OS reservation) that is cut off from the 256MB for the GPU. It’s the real bottleneck for this gen, even for multiplatform titles as it is the bottom spec to work with and is e.g. the reason why Skyrim has no DLC on the PS3.
The Xbox 360 is still the most efficient of the consoles with only about 30MB memory use for the OS (even now after many additions since launch) and shares all the memory between the CPU and GPU with the addition of superfast 10MB eDRAM (which in turn isn’t enough for full scene rendering at the highest resolution nowadays wihout tiling).

Even a USB2.0 port could in theory be used for an extra reserved caching drive (Readyboost like) to speed things up a little, yet none of the big three have really tried to use that as an option.. Who knows, maybe down the road…