I can't use modern static objects

[TheGreatCthulhu]

Oh, now it makes sense, but it's so... impractical to use Primitives to make walls.

Well, it depends. On one hand, plane-based surfaces don't require external mesh files, so the custom story / full conversion saves a bit on memory; also, since you can have planes of arbitrary dimensions, you can just use one plane in place of a row of 10 or so static wall-objects (you just need to adjust the texture tiling accordingly).
On the other hand, planes can, of course, only be perfectly flat, so if you want your walls to have features which require 3D geometry extending from or into the plane of the wall (like a lot of walls in amnesia itself), primitive planes can't support that.