Some Wikipedians have raised concerns that non-existent /Print subpages of high-use templates are not adequately protected. For instance, someone could vandalise the print version of millions of articles, by creating a /Print subpage of a protected high-use template. Sensitive pages such as the main page could also be vandalised, by bypassing the "cascade protection" feature which normally prevents templates included from certain articles from being edited. This vandalism might not be detected for some time.
See the URL above for related discussion. Various ideas for fixing this issue were suggested at that talk page. For the moment, I would recommend a relatively simple change: make the page protection of the parent template also apply to /Print subpage.
To do this, you would add hooks to Title::loadRestrictions() and Title::getTitleProtection(). These hooks would be intercepted by the Collection extension, which would determine whether the title in question is a /Print subpage, and if it is, it would re-enter the Title class to fetch protection levels for the parent page.