Space Marines should have one codex, in which, there are two - four pages of unique rules for special units, characters and army wide powers, for each individual chapter.
Games Workshop would theoretically lose money, but £50 for a mammoth codex Space Marines would make them a pretty penny and most players are for some chapter or another.
Black Templars, Crimson Fists, Dark Angels, Blood Angels, Space Wolves etc etc are all Space Marines, the only difference being their individual styles and their ratio of adherence to the index Astartes.
There is more difference between an Emperor's Children Warband and a Nurgle Warband, than between two loyalist Space Marine Chapters IMO. Even then, Chaos doesn't have individual codices (nor should it) so why do loyalists?
Its annoying, and if they removed a few of the seperate Chapter codices they could add two more xeno races or a mechanicum codex which is badly needed.
That's my opinion anyway.