There is a requirement(although the std. escapes me right now. Joe?) that fire ratings must be maintained in the event of penetrations in fire rated walls(attached garages would apply). This includes disallowing back-to-back penetrations in the same stud cavity, limiting the total area of the wall space allowed for device boxes(100 sq.in.?), and using metal boxes on fire-rated walls. If a metal box is required to maintain fire resistance ratings, a plastic plate seems to diminish its effectiveness.
