Omnis pick up sound from all around the capsule, so if you're stealthing in a loud crowd, you'll get a lot of crowd noise. If you're open taping with them, it's tough to run omnis from a section or in a bad room (since you'll get WAY more of the 'room sound' than with other mics).
getting up close and standing in the center (btwn L and R) takes care of some of those issues, doesn't it?
Getting up in the heart of FOB/DFC does help out with room issues, but does nothing for people yapping away, hooting, and yelling near you. Because of this I really dislike the idea of using omnis for stealthing shows in clubs, try it and you will be amazed at the sounds your mics will pick up, beers clinking, people blabbing away, cash registers, etc... Even just running cardiods ORTF I have heard stuff on the tape that I didn't hear live at the time.
For the person that started this thread, since he/she was looking for mics to use in small clubs I think omnis are not a good idea for that specific situation. If you are stealthing jazz or classical where people stay quiet during the music, they are more amenable, but I still prefer the rear rejection I get using cardioids, since even in these quiet situations, you can still pick up people breathing, coughing, rustling their clothes/programs, and so on. Of course, given no background noises (and hopefully, high SPL's) the little omni mics can sound great especially the DPA 406x's!!
Also, I think it is important to note that these mini mics typically have much less dynamic range than high end "active" capsule mics, if you are recording music with wide dynamic contrasts this can play a big part in the quality of your recording. I did some classical stuff with CSB>D8 that came out pretty poorly. Due to the high noise level of the mics and deck, the soft parts are very undefined and actually had some audible hiss. Now, this past summer when I taped the Chicago Symphony at Ravinia: DPA 4022>V3>Laptop at 24 bit, it was a whole different story!! I ran levels VERY conservatively (couldn't check levels during the music) but due to the the added dynamic range of this rig, the music still sounded great, with no apparent noise/hiss, even after some serious normalizing!! (I'm talking +15 dB or so) Just something else to keep in mind, in addition to size and polar pattern, dynamic range of your mic can have a noticeable effect on the quality of your recording.
Peace out-
Ben