In one of the clubs I frequent I could run on stage and FOH easily from the same spot.
4 mics can be a HUGE asset onstage
QFT
Some of the best recordings I've made, half the top 10 probably, have been 4ch onstage/stage lip.
I look at it in a big picture way though. Over time I've gone through several different 'main' sets of mics, but since I've kept almost every pair, I have different tools to use in different situations. I may run 4ch with the intention of mixing it down, or I may just want some insurance against pulling a bad tape (run cards and hypers) The sound I get from the LD capsules of the LSD2 are a great compliment with certain types of music, other times the addition of split omni's gives a needed boost to a narrow stereo image.
As stirinthesauce pointed out it is about flexibility.
I don't slap 4ch together because 4 is automatically better, but in the right situation it can give you a mindblowing result.
To your question about what your second pair should be, I'd say it should be something that compliments your existing gear well. It is more about tonal characteristics and pattern/placement options than brand.
With U89's I can't say that 140's would be a bad match, but experimenting and listening are the true test, which is easier said than done.
Is there something you feel your tapes lack right now? too bright, too dry, too warm? Pick a second pair that will mitigate that and round out your sound.
JAson