My experience is only with Pentax lenses, but based on my experiences with those I agree with everything you say, i.e. there are no adjustments so what's the big issue?
I've usually embarked on disassembly of lenses because of fungus growing within them (a problem which seems to affect Pentax lenses particularly badly - I have various lenses from Tokina, Vivitar, etc. and have never had that problem with those). Once I've taken apart a lens once (and made spanners for the threaded rings encountered) it's a simple matter to repeat the operation every couple of years as necessary. I do, however, restrict my efforts to lenses that I are relatively plentiful and cheap these days. For more unusual and expensive to replace lenses such as my SMC Pentax 24mm, I prefer to call on the services of Michael Spencer, as I did recently.

Steve