I'll go further on this, and I'm gonna put on my blunt-arsehole-infantry-NCO-hat and use the example of reserve NCMs in the combat arms.
A member is transferred to the PRL, yes, if they "cannot be conveniently administered through another reserve unit". Typically this is done when a reserve member gets a 2 or 3 year class B and ceases actively parading with that reserve unit. the PRL exists as an administrative organization to administer the need for reservists to work contractually for periods of a few years; it is not an operational organization that has a mandate to force generate, force employ, or to actively career progress.
So let's take two hypothetical MCpls:
MCpl Bloggins gets a 3 year contract working at a Cl B position in a major city. He continues to shwo up on Wednesday nights and parade with his unit as a section 2ic. He attends as many of the weekend exercises as he is able to.
MCpl Smith likewise gets a 3 year Cl B position in the same city. He elects to no longer remain active with his reserve unit. In order to free up that MCpl line serial that he is occupying but not doing the job of, he is transferred to a PRL.
So we have one which does contribute to a unit, continues to do his comabt arms job in a field environment, and continues to career progress within the unit. The other has taken a different route wherein the reserves are used as a mechanism to compete for long term contracts. He will not be employed in the environment of that trade.
Reserve units get barely enough positions on career courses for the advancement of their own people. I work ops in a unit with 126 infantry Pte and Cpl. Every single one who we wish to advance to MCpl must get DP2 Weapons Det Member and PLQ the whole way through. Every platoon weapons det needs guys to be qualified for the job, and we need depth, too. There is attrition at every stage. And this year I will be lucky if my regiment gets six or eight positions on the weapons det member course, which is run within the reserve brigades or at the area training centres.
So: If a reservist has elected to take full time employment for several years AND has chosen, while doing so, not to contribute to a unit, why should those very limited positions on career advancement trade courses be offer to them? An infantryman who elects not to work as an infantryman, in my mind, has no business expecting the infantry to give him positions to progress in his career as an infantryman, so he can then go and get another non-infantry Cl B at a higher rank level. We have ample full time Cl B guys who DO show up to train as well and who DO get those spots on career courses. MCpl Smith and MCpl Bloggins both hope to be Sgts some day. Which one, in their trade, MERITS the Small Arms Instructor's Course necessary to do so? We have guys working 3 year Cl Bs as Sgts now who did the same as MCpl and as Cpls, and did nothing more than keep coming out to play, and go on a career course every 3 or 4 years.
The reservist who doesn't contribute to a unit and who snipes that career course spot is boning the guy who does. Moreover, that reservist who has been employed for some years as an NCM and has not worked in the section, detachment, or patrol is unsuitable to be advanced to the next rank in that same trade when they have not retained, refreshed, or further developed their combat arms trade skills. You could take an RMS clerk who hasn't worked in an infantry section in four years since they changed trades and put him through a PLQ Mod 6 Infantry. Doesn't mean he's got any business claiming to be an Infantry MCpl afterwards. How does that differ fromt he reservist who goes PRL and ceases to do anything in trade?
If a combat arms reservist wishes to work full time, not participate in a field force unit, and to career advance, the answer is to component transfer to the regular force in a suitable trade. There he will have the opportunity to work full time and to advance with only the very reasonable expectation that he be subjected to full time career management.
Alternatively, show up for the three hours a week and one weekend a month that is all that is expected of a reservist in order to be merited for that career advancement. I am not aware of any reserve combat arms units that don't have some vacant line serials for NCMs to transfer into. Contact a unit recruit, express interest in transfering from PRL back onto a unit establishment, and demonstrate that a spot on those courses is merited not arranged through a back door deal where a reserve brigade is obliged to surrender a spot on a course they're running for their troops.