I have set the on-the-fly to "ask me first". If I query with kpsewhich a package which is not installed on my system, eg. bg.sty from the backammon package with
then the on-the-fly-installation dialog pops up and asks me if I want to install the package. That's a problem if kpsewhich is used in scripts to get the location of files. Imho kspewhich should not by default start the on-the-fly installation.
(I know that kpsewhich is not the official tool to find files, but it makes life easier if you have to write scripts meant for texlive and miktex).