In reply to Lloyd Entwistle:
Wrote a longish reply and then lost it...
I think that triple ropes are often only just within one of the standards. Up to you whether you're bothered about that or not. Also as others have mentioned they are often more expensive.
E.g. The Beal Opera you mention. In the single rope test it only achieves five UIAA falls (ff1.77, 80kg mass) before breaking. There are 99 single ropes on ropeparison, 90/99 of those achieve more than 5 in this test. There are 21 triple ropes on ropeparison, 10/21 achieve more than 5 drops before break. Clearly (and probably unsurprisingly) triple ropes are on average 'worse' at this test than single ropes.
E.g. The Edelrid Swift Protect 8.9. In the twin rope test it has an impact force (ff1.77, 80kg mass, two parallel strands of rope) of 11.8kN, only just inside the max allowed by the standard of 12kN. Dedicated twin ropes and most half/twin ropes have a much lower impact force in this test.
Obviously those two examples don't prove that "triple ropes are often only just within one of the standards", that's just my opinion based on looking at some graphs in an informal manner.
Attached a graph of twin certified ropes (triples, twins and halfs/twins), showing sheath percentage and impact force. (Unfortunately it's not really obvious on the image which are triples/twins/halfs, need to make them distinct colours at some point.)