I whole hardheartedly agree ... I set up the environment for them to be inviting and promote and develop certain skills and they engage in it and manipulate it while I 'observe' and make changes to continue to stimulate and scaffold their learning to the next level.
I am more inclined to be found sitting back and 'observing' play with my camera and a note pad verses being 'engaged' in their play to be honest specially when they are 'all' here cause there are enough of them they do not NEED me interrupting or changing their play by participating in it - the only time I would step in is if a 'disagreement' is brewing and after giving them pause to see if they can resolve it on their own it is still escalating - than I would step in to help them problem solve by asking questions and getting them to brainstorm solutions until we find one we all agree on or if a child approached me and invited me into the play - and than depending on the child I might join in OR I might redirect them to approach another child with that invitation (for those kids who seem unable to play with peers to encourage them to develop that skill).
Cultivating and supporting the development of IMAGINATION is the best thing we can give children - it leads to creativity, problem solving, risk taking, greater understanding of how their world works around them and so forth ... I often tell parents that if children needed to be 'entertained' the minute we gave birth we would transform into CLOWNS ;)