That was seriously uncalled for. The AM's point was good, and I must say that had I run across a forum where someone was posting on why host variables are a good thing, I would have been left similarly confused. Yes, you can search. But you shouldn't have to.
Remember that general Perl knowledge does not imply knowledge of various APIs. And I know of top-flight Perl programmers who have never used SQL. If someone has used SQL extensively, but from C, then being introduced to a new vocabulary to understand an old concept is going to leave you unhappy with good reason.
Incidentally host variables are not quite placeholders (they use the same API in DBI, the syntax is a tad different - :foo vs ? - and the placeholder syntax is more portable), but they are close enough to get the point.