I know someone who used to have a PPL/FI, and it worked very well for him for a short while.
He was an experienced PPL and a full-time ATCer. He fancied doing a bit of part-time instructing in order to get some free flying and improve his own skills. He arranged with my boss at the time to work part time for the school I worked for at the time.
It worked very well. My boss was switched-on enough to realise that if he gave all the work to the "free" part-timer, his full-timers would soon start getting upset, so having this guy working part-time didn't affect the amount of work I or my colleagues got at all. He got to fly for free on his days off. He was certainly not considered an "apprentice" - he had held a PPL for many years, and had vastly more experience than a lot of newly-qualified FIs.
After a while, he decided that the extra effort to get his CPL was well worth it, since getting paid for instructing would soon pay for the cost of the CPL. But, for the short time in which he instructed without a CPL, it worked very well for everyone concerned.
FFF
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