Older (pre-MagSafe) Mac laptops actually do something similar to what you suggest; the last generation of PowerBook G4 took a 65W supply, while older PowerBooks had a 45W supply but used the same plug. The 65W would naturally power the older units, while the 45W would power or (but not and) slowly charge the newer units; if you were using it heavily enough, it wouldn't even completely power the laptop and you'd drain your battery, just not as quickly as if you weren't plugged in at all.

Apple put a sensing mechanism in so that the laptop could tell what level of juice it needed. Because of this change, I needed to get a new tip for my iGo aftermarket adapter; it's a 70W unit, so it could easily power the newer laptops, but the original tip only "sensed" as a 45W resulting in a lower draw and limited functionality.