Captain hit the nail on the head.
The air barrel in the R109, R156, G250, etc is quite large in relation to the poppet diameter. The balanced poppet in the R156 and G250 are supported by the lever and the balance chamber while the unbalanced poppet in the R109 is supported by the lever and the spring. The only thing that may contact the air barrel itself is the outer edges of the spring and any movement there is minimal as the poppet and spring move very little.
In short, even if it is worn to bare brass in there (which is quite likely and quite common given how they were plated, any corrosion that occurs on the brass is going to be minimal as brass is itself corrosion resistant and is not going to impede the function of the poppet or spring. In particular, anything that forms where it may contact the spring will be constantly removed by the spring before it is problematic.
If you are the original owner of the reg, ask the dealer who is telling you your reg is worn out and no longer servicable about having it replaced for free under warranrty with a new G250V - which would be the current model comparable to the R109. If you are not the original owner you don't have that option and may want to excercise the "find a new dive shop that is honest" option.
In the past I know that at least some Scubapro sales reps have given shops a great deal of grief in regard to servicing older regulators as they felt the dealer should be selling those customers new regs rather than servicing the old ones (Mk 5's Mk 10's, R109's etc).
One of the perhaps unintended consequences of building really good regs that last and perform very well even after decades of use is that customers really have no legitmate need to ever buy a new one and Scubapro is dealing with that particular problem with some of the really exceptional regs they produced in 1980's and 1990's.
One thing I have noted with very old R109's is that the levers used in the early versions (late 1960's to 1970's) have slightly wider tabs that do not work with the current S-wing and Duro poppets. However the current G250 lever works just fine and is about a $5.00 part to replace. It's possible that some of the roughness they may be feeling in an assembled R109 with an old lever and new poppet is the inadequate clearance betwene the two. It is important to remember that SP does not teach techs about old model regs at most of their service clinics so unless they have an experienced tech who has been around long enough to know how to work on them, they don't have much opportunity to learn the finer points of them. So it may be ignorance rather than dishonesty, but neither is a great trait for a dive shop to have.
If you replace the diaphragm and the lever and upgrade to the balanced S-wing poppet, the R109 Adjustable and R156 Blanced Adjustable breathe just as well as the current G250 and S600. The older brass orifice is far superior to the current plastic one, so unless it gets damaged be sure to keep it rather than "upgrade" to the new plastic version.
