Beware of oversimplification re: the "flatness" of any omni; it's not a property of a microphone by itself, but of the interaction among the sound source(s), the acoustics of the recording space AND the microphone. How "flat" an omni is, depends (among other things) on how it is placed in relation to the sound source(s). An omni can be flat for direct, on-axis sound ("free-field equalized") or for sound that arrives from all angles equally ("diffuse-field equalized"), or for something in between. But an omni can't be flat in all different types of placement, unless it is so small that it would be rather noisy.
I happen to use Schoeps microphones primarily, and their product line illustrates this nicely. Please visit
www.schoeps.de and look particularly at the curves for the capsules MK 2, MK 3, and MK 2S (or MK 2H as an alternative). The MK 2 fits the first description in the paragraph above: flat
for direct, on-axis sound and thus very nice for studio recording, or for the occasional concert recording in which you would find it suitable to place a stereo pair fairly close to the sound sources. The MK 3 fits the second description (flat in the diffuse sound field), but it is rarely used for stereo recording, since it's really designed for more distant placement than you would usually choose, assuming that you have a choice.
The MK 2S and MK 2H on the other hand represent the type of omni typically chosen for a main stereo pair; they're at carefully chosen points between free-field and diffuse-field designs. Their published graphs show a rise in on-axis response at high frequencies, but the microphones actually
sound flat when used at moderate distance from sound sources in typically reverberant performance spaces. That's because there is absorption of high frequencies in the reflected sound, and that reflected sound makes up about half (or perhaps somewhat more) of what is being picked up at those distances.
--best regards