Dave,
Getting back to the thread late, you obviously have a point that tube-rolling has questionable merit depending on your PoV and what's intended.
There are really two motivations: system tuning and better sonics overall.
The first case is a matter of practicality - everything is colored and putting together a completely neutral system from the get-go (or not if that's not what you want) is not all that likely. So you tune - with tubes, cables, etc. In this case, the manufacturer obviously can't pick the correct tubes.
The second case applies to tubes of certain types and variants that simply sound superior to all ears, or at least have certain standout qualities. In these cases, the maker often *could* provide "the best" or at least better-than-chinese-stock tubes and sometimes why they don't is a slight mystery. (Cost and availability are obviously the answers..)
However, it is not true that tube-rolling is always an option: it flat-out isn't with my Shindo preamp. Shindo-san owns essentially all of the signal tubes of this type there are, from what I understand, and there is really only one variant as well. To your point, he *does* very specifically voice the unit to sound as good as he possibly can, and he don't want nobody screwing with those tubes. In this sense, these unit are no different from a SS counterpart at all. Since they do use tubes, which are naturally far more linear devices than transistors (except JFets??), their circuits are simpler, parts count lower, and heck if the best sounding pres don't all seem to be valved.
