cavernut, the directional pattern specified for a microphone is a generalization, an incomplete description. The actual directional pattern of nearly all microphones varies at different frequencies throughout the audio range. Not only does this affect the way they relate to the sound in a room, but it also means that sound sources at various angles off-axis will be picked up with varying sound quality--sometimes drastically so--because the frequency response of the microphone depends on the angle of sound incidence.
Stereo recording with two main microphones relies critically on the off-axis response as well as the on-axis response of microphones, so it is generally done with smaller microphones. Of course these are also easier to handle physically, and block other people's sight lines less. But the main reason is that consistent freqency response both on- and off-axis requires either small pressure-gradient (i.e. directional) microphones, or if you want to use omni mikes they have to be very, very small (which generally means they will be noisy). No large-diaphragm microphone ever has anywhere near the same frequency response curves on and off-axis (i.e. the same sound quality in all directions); it's a physical impossibility because of the size of the mike vs. the sound wavelengths, unless you want to give up all meaningful high-frequency response.
I suspect that the next step in your audio education ought to be to learn to read polar diagrams for microphones. Neumann publishes good, clear ones; go to their Web site and compare those of the TLM 49 or M 147 for example (both microphones based on a "classic" large-diaphragm capsule design) with those of (say) the KM 183 and KM 185 and you can learn a lot. I'd be glad to talk through it with you here.
--best regards