When you crank a monitor in a room you already get some 'monitor in the room' effect and maybe that's a good target as well. When you have to much 'cab in the room' ambience and eqing in your signal you hear the room twice, the real room where your monitor sounds and the captured room as part of the signal, one room too much.
That is what I think as well. Additionally, using the "null" mic setting seems to work well (for me at least) when playing in places that have a ton of natural "ambiance" because of the construction. On two occasions I swapped to a different IR because of the effect the physical space of the venue had on my tone. Being able to do that is pretty handy.