In a given channel, 802.11 mesh networking provides roughly 1/7th the performance of non-mesh (and an ideal protocol only achieves 1/3) [1]; using more spectrum increases throughput but the waste is still there. Many people (especially telco lobbyists) would argue that spectrum shouldn't be wasted on such inefficient protocols.

[1] http://pdos.csail.mit.edu/papers/grid:mobicom01/