Post-apocalyptic doesn't necessarily mean unstable. We weren't post-apocalyptic back in the 1930s or 1950s, and computing machines had plenty of uses already (and even earlier).

> Post apocalyptic societies have zero practical use for computers.

Post global nuclear war reconstruction would really benefit from having even a simple spreadsheet program to aid with resource allocation, wouldn’t it?

> Whatever you do with them [computers] is automatically seen as practice of an ancient and unchanging tradition.

During a nuclear war reconstruction scenario, computing wouldn't be seen as ancient or unchanging. We'd be in a transitory period where we try and recreate computing infrastructure with what we have and the power sources we have access to. But we wouldn't develop an ancient tradition or anything.

At lot can be done with pen and paper but if you were rebuilding civilisation from scratch I'd be be happy to have a solar powered computer holding a copy of wikipedia and heaps of documentation to help me put to use any artifacts I'd find from the old world.