The reason for having a policy on the MAN interface, assuming it's physically a gig connection, it allows you to shape at your contracted rate, which allows you to "know" and "manage" congestion for the 300 Mbps that you otherwise could not.
As to using policers rather than shapers, both will drop packets, the question is when. By default parameters, policers generally will drop before shapers, but based on parameter settings, they can drop at about the same point if desired. (BTW, also by default parameter settings, policers often tend to drop sooner than a like speed interface would - remember interfaces buffer too.)
Generally, I dislike policers as they will drop traffic from a short duration burst, that a shaper would past, but one thing a policer will do, it will avoid latency variations that a shaper (or interface buffer) will create. Also, policers can be used in both in and out polices, shapers only in out polices.
