I think that's a very good point in some contexts. Assembling and disassembling things, it often makes them go wrong or wear out, especially if they are engineered to very fine tolerances as modern machinery often is. I used to rebuild crossflow Ford engines in Mark 1 and Mark 2 Escorts, A-series lumps and old XKs all the time, and with some love and care it was not very hard to make one that fitted together better, ran smoother and lasted longer than they came out of the factory. But there's no way I'd take apart the major moving components of my 94,000 mile BMW diesel straight-six, even if I had the facilities. I might in the fullness of time replace the injectors but I don't think it would be improved for a hand rebuild that included re-ringing, replacing the main bearings, or any of the things we used to do quire routinely. I'd never be able to hand-build an engine of that power and economy to be absolutely oil-tight and emissions compliant the way it is now, it would just not be as good as it has become by running-in from a factory build. But for lower tolerance old-fashioned things, pre CNC tooling, machines characterised by deliberate oil consumption, big parts and grease nipples everywhere, they can usefully absorb quite a lot of maintenance without being seriously disassembled, and indeed be designed to receive it.
There's a really good article on "disturbing things" by Bud Davidsson, here:
http://www.airbum.com/grassroots/GrassrootsKarma.html