Murray Mac -

No argument if you choose to glue them in. Lots of luthiers do, as I commented. Lots of others don't.

I'll agree to a difference of opinion, but as to "wrong" it just depends on who you're working with.

For someone trying to "fix a fret," assuming especially that the fixer is of "amateur standing," I still would prefer that they omit attempts to add "glue" if they plan to bring it to me to do the repair on their repair.

Cyanoacrylates and epoxies generally are NOT SOFTENED by temperatures that don't char the wood, or at least boil off the volatiles and degrade the wood in the heated area. In general, within practical limits, they don't soften when heated until you get hot enough to cause chemical breakdown. The wood can be softened, and it may look like the glue softens and comes out easier, but heating the fret to acceptable temperatures has no significant effect on most of these glues, and especially on some that the "DIY fixer" might use since there are no reliable performance and quality specs on what you find around the house.

And if glue has been added, you still have to clean "the rest of the glue" out of the slot if you want a good fit for the new fret.

One of the nice things about Luthiery is that the luther is permitted to do whatever (s)he thinks best, and quite often things having no real or rational basis in science or music turn out quite well.

John