The few occasions where I teamed up with fellow multiboxers were quite disappointing. The either showed up completely clueless about the encounters, unprepared in terms of gear (no gems/enchants when they were costing a couple 100g) or macros ... while I had different expectations/made pretty much all preparations possible. I wasn't really interested in raiding content that was 2 years old by severely outgearing and bruteforcing it. I have however taken 4 shamans into a normal raid back in cataclysm with my guild back then, but that didn't really feel right as this was a heroic progression guild (i.e. it was more for the gimmick that a boxed a few encounters than actually contributing).

The other possibility would be to 10 box: while it means I have to make some changes to my setup (don't really fancy bringing e.g. a 2nd computer into my living room) I've always considered 5 to be the ideal size of my team. And what stopped me from 10 boxing in cata was that a simple trivial raid mechanic on boss 2 of an instance could make it impossible to further progress, which would make a 10m team pretty useless for me as there is nothing else to do on your own.

And this might sound crazy, but I multibox raid encounters in my head all the time. For pretty much every boss I think about how a multiboxer should do the encounter. Then I check logs/fights/raid comp till I'm convinced it would work (and I have a pretty good background as a progress raider, so I somehow have faith in my own analysis). And the knowledge that the strategy I come up with would work is so far enough for me that I don't feel the urge of going through the hassle of actually doing it.