Well, most teams did then too, but your team hasn't ever even played the current template, though that's not to say you couldn't have similar success on the current template sans mirror picking, but I'd guess in general that the more picks a team has the more mirror picking becomes ideal, so it's probably more beneficial with the current template than it was when you and peixoto were playing on the ladder.
With two picks the pattern was easy. First pick in a nice bonus, second in a two turn +4 bonus and third to counter you number 2. Getting a fourth pick was relatively unlikely just as losing your number 1.
In the 3v3 ladder mirroring is the obvious solution when not planning something special like a Poland clusterpick. If you don't mirror and your opponent mirrors the islands it can be game over right after picks with his picks pushing you out of a complete area.
I'd say the reason people tend to mirror pick on a template like the current ladder template is that coverage tends to be better than combos. In a template where this is the case, mirror picking is usually superior(assuming you pick decently well). In my experience, mirror-picking is the best way to get coverage, since you can prioritize the important areas twice as well as when you don't mirror pick. Not mirror-picking has it's own advantages of course. Making sure one person has a certain part of the map to themselves is a good way to ensure that your expansion doesn't interfere with your teammate's, allowing for faster income on maps where there might not be that one superior pick.