I think its best to take powerplay when you are really going for the sixers or aerial shots i.e. end of the innings. But then again you actually need to have guys at that time who can do that, having number 10 and 11 at the end of 45 overs, and if powerplay starts then its going to be useless. I think Sakib needs to assess after 40 overs if we are in dire straits and dont have wickets with someone like Naeem stranded in one end, its better to take the PP then.
When The Going Gets Tough, The Tough Gets Going.