I think scales are overated for technique maintenance as their actual presence as such in the repertoire is pretty small so a lot of effort is going into practising a skill that is narrow in it's focus and actually is necessarily not going to be used terribly much!
You know its funny, you certainly are right, a majority of the repertoire don't have long 2 and 3 octave scales. So should we just never practice them? What about those few instances where we may (at some point in our lives) encounter them...shall we wait to develop that technique when that day arises to play a piece like the Chaccone?
While I am not in the age group being asked about in this thread, I'd like to think that I would be able to partake in the conversation at hand.
This idea of developing and maintenance of a technique you may never use ever in your life reminds me of discussions I'd have with rock guitar players. Guys saying, "Why do you need to be able to play this fast, or do this lick, or any other technically challenging idea on the guitar?" I always answer with this responce, "Because I don't want my technical limits to limit my musicality." The same goes for classical guitar, if not more. Perhaps because its fresher in my mind, but I remember quite vividly my playing and how it was effected by NOT practicing scales on a regular basis. I have recordings to prove it (and not in the good way). I also know how much easier it became to play the instrument as I started to make scalar practice part of my everyday routine. We aren't talking about the development of speed here, but rather just the ability to just play the instrument and the sound being produced. How easy my fingers would work even outside of the realm of 'pure' scales. The change is almost instant, the fact that everyday I was constantly refining and watching over my technique meant that it was kept in check on a regular basis.
Sure this can be done with musical excerpts. And I also use "studies" either from pieces that I'm currently playing OR from old/new repertoire that I'm not planning on doing in concert. However, what I've found by ONLY using musical examples is that (at least for myself) I'll get too preoccupied by musical issues at times with the piece that some of the technical things can and will be missed. This is why I work with the 'boring' scales. And this concept is constantly evolving, and JUST changed for me this summer. I'm no longer working up small burst groups, but rather for 20 minutes with rest, then 20 with free I'm going through my whole burst pattern, going from 5-note groups all the way through a 2 octave scale. I can tell you that by doing it for only one week with HALF the time spent on it (I was doing 20 minutes for the whole "workout" rather then 40) that my scale technique has improved. I'm continuing on my quest to be able to play a 3 octave scale all the way through without feeling like I've left my fingers in the other room, but I've lowered the amount of time to 10 minutes for each stroke. Sure, this is a FULL hour on 'boring' scales alone. But the one thing I've found incredibly useful is to break up the hour chunk throughout the day, almost as a way to wind down from working on large works (things like Bach and the Rossiniana can kill a man). I'm also using currently Etude 7 (Villa-Lobos) as my scale study, and when that's completely, I'll probably either pull out some of the smaller scales within the Rossiniana or excerpts from the Chaccone or the Aranjuez.
Anyway, to get back...a little bit...onto topic, I think that some kind of regular work on scales in a setting that isn't going to distract you from the task at hand is incredibly important, and I would think for the aging guitarist even that much more important! I think of it as a daily workout for my technique/hands so that they are in shape. 10 minutes of basic scale work is better then nothing. Obviously the more you do the more you'll get out of it, but then it becomes a practical thing. Some people don't have an hour to devote to scales. It just so happens that for me currently, I'm able to do so, so I am. We'll see what happens as the school year comes along to kick my butt. I think this regular maintenance/workout will only help you to be able to refine what you've already learned and keep your eye on it. In my short while as a guitarist, I've found nasty habits that I thought I corrected come back to bite me in the least desirable times. Never take anything for granted, and never assume anything about technique, that's what I've come to learn. Treat your fingers and hands like they're kids...and if you do so, unlike kids they'll listen!