Thanks Richard, you got me thinking through this again this morning.
As you say, Blumlein array is equally sensitive to sounds arriving from any horizontal direction. I agree that's one aspect that lends a certain naturalness of capture. That along with other non-stereo based perceptions of spaciousness which come into play may be what your are considering. For example, in the monophonic case, an single omni will pickup reflected sounds arriving from all directions and those sounds give us information about the size and shape of the room the mic is placed in. A single more directional mic attenuates those off-axis sounds and sounds less spacious in the monophonic sense, trading ambient reflection information for more direct pick-up. There may be other aspects that also lend a sense of spaciousness as well, but what I'm mostly referring to here is a sense of stereophonic spaciousness.
Consider that two coincident cardioids angled 180 degrees apart are also equally sensitive to sounds arriving from any horizontal direction as a stereo pair, just like Blumlein. So are two coincident omnis. But the degree of perceived spaciousness decreases with each case. It's the difference information between the two signals that lends that stereophonic sense of spaciousness. In these coincident examples that difference information is comprised solely of intensity differences.
Once I space the mics apart I get time of arrival differences as well, depending on the direction of the sound source. There is no reason you cannot space a pair of figure-8 mics, but in doing so you may want to adjust the angle between them at the same time. It's the Michael Williams Stereo Zoom thing- the relationship of pattern, angle and spacing between mics. If a Blumlein array has a SRA (the Stereo Recording Angle of pickup in which your source is located) of around +/- 40 degrees, then spacing the mics without changing the angle between them would narrow that SRA further. To keep the same +/- 40 degree SRA while introducing some time difference information you'd narrow the angle between the mics as you space them further apart. For your 15cm suggestion, that angle would be about 62 degrees (see the chart below). Continue on in that way and you'll eventually end up with two figure-8 mics spaced about 60cm apart facing directly forward and using only time differences for stereo pickup, acting like spaced omnis in that aspect. Of course they'll sound different for a number of reasons, a big one being that they will reject all sounds arriving from the sides (and form above and below) which might be useful. The Tony Faulkner array of two forward facing figure-8s is this type of setup, though he mentions a closer spacing between mics which would widen the SRA from the above example. Stereo Zoom is just one approach, but conceptually I find it helpful. One detail to consider is that for anything thing between the Blumlein and fully forward facing configurations, sounds arriving from the rear of the array will have contradictory time and intensity differences. I don't know if that would actually make any difference or not in a real situation.