I usually come across people who don't actually know what a passive is[1] or who are completely and rabidly anti-passive, but I don't doubt that such advice is common among more reasonable grammarians. Still, there are sentences that are difficult if not impossible to rewrite in such a way because of context or lack of vocabulary, e.g. "... and the network interface is written in C," is difficult to rewrite because the old information (the software) comes first, the new information comes at the end, the unnecessary agent is omitted, and no active verbs come to mind.

[1]: http://languagelog.ldc.upenn.edu/nll/?p=3884