Coat tail effect
To restate Mark's point: Lunar exploration can ride on the coat tails of a manned Mars program. However a manned Mars program can not ride on the coat tails of a manned lunar science program. Also a manned Mars program is sustainable provided the political will exists for Space Exploration (the real problem). However a manned science base on the Moon even with lots of political will would probably time out after after one-or-two decades. There isn't that much one can do on a low-gravity volatile-free ball of basalt except do radio astronomy on the far side.
It's all about having cakes and eating them.
Gary