Multicast

Hello,
First the CBT protocol was created to use shared tree solutions because
DVMRP and the other dense mode protocols werent scalable. there were
many problems with CBT (which is bidirectional) so PIM-SM was cretaed
which provide some switching (between shared tree and source tree). and
after that there is some discussions about the bidirectional PIM, which
is like CBT.
Are we in circle here or what ??