[derefr]

I do wonder if the doors—at least at first—were more supposed to be metaphor than prop; given that you have to blast them open, perhaps they were just an indication of passibility to the player, where Samus would just see rubble or rock walls or rust-frozen mechanisms needed to be blast-cleaned, etc. One visual symbol standing in for many scenarios.

[fixermark]

They're a necessity.

The original Metroid needed them for expensive loading transitions between areas, as palettes and the scroll buffer were swapped out. Interestingly, they also serve that purpose in Prime... If you've ever shot a door and had it change color to show it unlatched but refuse to open for several seconds, that's the console stalling on loading the area behind the door.