Yes. I keep the jag on my range rod all the time. For most of my guns, I just have the brass tip on the guns' field rods, though some have a jag, some just a wooden-ended rod. Most of my guns have NEVER been loaded with a field rod. Reason; practice and informal "shoots" are at (formal) ranges, and that is 95%+ of my shooting. Bringing a range rod along isn't a big deal.
The brass jag I carry in the PB (along with spare flints and other less-needed items) can be threaded in to the end of the field rod if needed.
The reasons I don't leave it on the field rod is several-fold;
1.) A clean smooth brass tip looks better.
2.) A clean or bare end is more historically accurate for the period of guns I mostly use.
3.) My tipped RR is the full (internal) length of my barrel already, so with a charge, I don't need the extra length the jag would lend, and I don't want it sticking out past my muzzle. If I ever have to pull a ball in the field, I want all the RR I can get to twist the ball-puller screw in to the ball.
4.) On recoil, the RR typically moves forward just a bit. The jag might snag on the end plate of the MC and want to dislodge it.
5.) I NEVER use a field rod for cleaning, and the jag part is mostly for retaining cleaning (or swabbing) patches. I don't swab between shots in the field.