I think they do it quite often on these special occasions precisely because fangirls scream in disgust / joy at the same time. That's why they try to re-create the TOP/Hyori spectacle all the time. Extending that to a promotional period or career is riskier and more difficult. Doesn't help that some of the idol co-ed group attempts have been so very terrible, of course. 'Trouble Maker' is a jam and one of the bigger hits of 2011 and I wouldn't be surprised if they put out another single as a unit in a year or so.