morst, the original post had asked about the MK 22. It's the MK 21 that's halfway between omni and cardioid. The MK 22 is between an MK 21 and cardioid (!).
The MK 21 was introduced in 1988 under the awkward name "wide cardioid". But it behaves more like an omni that just happens to be 6 dB down in the back. In terms of "tone" and "feel" of the recordings it makes, it has about 80% of the aesthetics of a good omni, including the seemingly limitless low-frequency pickup and spatiality. A pair of them can be used in spaced setups as if they were omnis; the result will be somewhat clearer because there's somewhat less room sound.
But paradoxically, at high frequencies the MK 21 has a broader pickup pattern than an omni; for reasons of physics, small pure pressure transducers become "beamy" above 6 - 7 kHz while the MK 21 does not. This means that the wide cardioid doesn't become much darker sounding with increased distance in a reverberant space (where the proportion of sound dulled by multiple reflections increases). And due to their directivity, pairs of "wide cardioids" can be used with relatively narrow spacing between them -- a foot or two, for example -- without any "acoustically opaque" object between them such as a Jecklin plate or a sphere. This gives excellent spaciousness and stable localization simultaneously, which widely-spaced microphones of any kind can't deliver (omnis least of all, sorry to say).
All in all, I think that many people who record with spaced omnis as their main stereo pair would be considerably better off using high-quality wide cardioids instead.
The MK 22 came along some 20 years later. In a more rational universe, in retrospect _its_ name should perhaps have been "wide cardioid" since it really is a variant of cardioid with a somewhat wider pickup pattern, just as supercardioid is a variant of similar degree in the other (narrower) direction. It offers a warmer-sounding alternative to standard cardioids, in that its off-axis response is almost entirely free of any high-frequency rise; at 45, 60 or 90 degrees its response curve runs almost completely parallel to its on-axis response. That's also true of the MK 21, but the MK 22 has greater directivity, so it can be used somewhat farther from sound sources without losing focus.