I think your (both your) sample sizes are too small for your generalizations. I just spot-checked 4 different PowerShot models using LiIon batteries (SD990, SX230HS, S110 and SX60) and 7 EOS models (D60 [not 60D], 20D, 1D3, M, 5D3, 7D and 7D2); with the exception of the 1D3, where the battery is the battery door, all had metal along the inside of the door. Clearly, "high end" cameras do have metal on the battery flaps; whether it's there for strengthening or not, I couldn't say; it's clearly not required to serve as a conductor for the battery in any of those models though. It may be for the switch used to turn the camera off when you open the battery compartment; all these cameras will turn off when you open the battery cover (or the card door, but that's another story).
FWIW, the A620 and A1300 I looked at, as well as the GP-E2 and assorted flash units (all using AA cells) also have metal lining the door; in those cases, dimples on the metal make it clear that they do use the metal as part of the conductor for the battery power regardless of any other possible uses.
BTW, Colin, you might want to look more closely at your battery doors; in at least one of mine, the metal had been coloured black.