Why not? With 3-6-1 the full backs can support the attack a lot more, and even help in the midfield. You might not even play with full backs, but with wingers with Willian/Ramires style, who will support the defense when its needed. For example, Brazil won 2002 WC with 3-5-2 (some people say it was 5-2-1-2, or 3-4-3, but in Brazil we always considered that a 3-5-2 formation). The full-backs (Cafu and Roberto Carlos) were free to support the attack, they helped in the midfield a lot, but when things were ugly the team could easy adopt a 5-3-2 system. I think its funny you said it might be risky, because I always saw 3-5-2 system as pragmatic one. Holland was a pragmatic side in World Cup and they took few risks (and think they were more 3-6-1 than 3-4-3 as some MU fans are claiming). 3-5-2 or 3-6-1 might be the perfect formation if you have the right players, for example, think about a full back like Marcelo, Dani Alves or even Azipilcueta, they might perfect for that system, but the only position that will suit Ivanovic in that system will be as one of the CB. He doesn't have the skills required to play in the mid, or to support the attack in the proper way. Thats system will suit players like Ramires/Willian as well, since one of the reasons Mourinho love both players is because of their defensive work rate.