I'm at a rare loss of words.  As a question of economics, a professional
technician needs to do what his customers want, but as a matter of
practice, I would say that the default when the piano is in the shop
should be doing the method correctly.  Looking at the investment of
time in a restoration, it's a trivial addition to correctly replace
keytops with hot-glue and clamping.

A long term implication: if a piece of ivory is glued on with Krazy
Glue then it's stuck there.  What happens if that piece of ivory
becomes chipped?  How do you remove it to repair it again?

How would you like to repair a player where the pneumatics were glued
with Krazy Glue?  It would be easier to write it off and move on to the
next one.  I know that the comparison isn't fair because you don't have
a guaranteed need to replace the ivory again (as you do with pneumatics),
but when the need does arise, I'd much rather be working on keytops
that were glued on with hide glue than with Krazy Glue.  Remember,
when Mom's not looking, that Junior still has his Magic Marker pens,
and remember that small hammer that he got for Christmas?!

Best regards,
Eliyahu Shahar