I can only speak for myself since I am only myself, but I might as well put my comment in.
I'm currently a JASM for my troop. Last year, my troop was suffering from sufficient leaders and we were struggling with keeping things in line, scouts and programs alike. Our SM was new to the job and was there mainly to revitalize the troop again, something the old SM could not do because of other matters. In short, my troop needed support leaders.
I had already served my troop in junior leadership positions, from patrol scribe to troop scribe, APL to SPL, and frankly, I was getting weary of it. I was always dynamic and made changes, especially when I was SPL, and introduced things otherwise alien to the troop, like uniform inspections and the Instructor position. So, before camporee and summer camp (which I would staff), I asked my SM if he would be interested in me serving as a JASM-- an offer he readily accepted.
Note that I was not suddenly given the job, but rather accepted it. It was not because I was forced to do so (I actually drag my mom to all my Scouting events), but because my troop needed me. I also was admittedly not exactly the most effective go-to leader, using slower and more methodical approaches to issues, though I always got the job and more done. I was ready for a more advisory job. After summer camp, I was given the position. It was almost funny how nobody in the troop, not even the heavily-involved ASM/former SM, knew much of the position! However, I did get the job. And frankly, I'm loving it.
I've eased into a mentoring capacity and serve as one of the main leaders in outdoor activities, which I am most proficient in. It could be said that outdoor events are my area in the troop, also keeping a handle in insuring that Scouting methods and protocol are kept, even advising the committee on how to make the troop better (remember, I'm dynamic). I do not feel like my position is one that is a retirement age for the senior scouts, I rather feel that I am in a better position than ever to help make the troop better. I don;t feel like a scout, but as an adult, and think my current position will certainly make me better as an ASM in the near future. And others tend to think more highly of me when they see my POR patch, cuz it's not one you see a lot in my area. I make friends out of commissioners and execs and feel listened to. I don't feel excluded from the Scouting system at all.
I'm the multi-purpose minion, so to speak. While I am aware that not every Scout (or JASM) will be the same as myself, I do think I set a good example for the job.
All in all, my view on the JASM is for the Scout to serve in any way he excels in best, whether it be assisting the TG or SPL, or being in charge of camping and hiking activities, like me. Treated as an adult, being a part of the adult patrol (if it exists) and operating in an advisory role. And if anything, the JASM would be in a great position to ensure the troop goes in a boy-led direction, since he has the most recent experiences and is still a boy himself (and is thus less disillusioned than the old geezers... just kidding).