So, maybe you'd put multiple mics in a single location to have an option of which sounds better...perhaps you're not sure of the configuration to use or which mic might sound better. Also, have two rigs flying can provide some redundancy in case something happens to one of your mics.
But in terms of mixing them together in post, as acidjack states, my experience of doing 4 mic matrix recordings in post rarely turned out well for my ears because I'm into definition and if the result is muddier, that's not what I'm after. The mixture was typically less defined and more muddy than if I'd left the pairs separated. Sometimes the overall sound might be pleasing to some people because omni's would make the recording sound fuller and sometimes warmer, but definition was sacrificed. So, in that case, the conclusion of whether it sounded better depends on whether you are a bass freak or a detail freak.
In the end, I stopped bothering because it was just too much of a hassle and I wasn't getting anything back out of it to justify taking the extra gear.