It's not needed in any strict sens of the term. I even have the impression
that quite a few Portfiles that do check for other OS types (I noticed
FreeBSD the other day) don't mention this in the platforms statement.
I wouldn't mind adding the info, but it can give the impression that it's
non trivial to use the port on another OS or even that there's official
support. Neither of those impressions would be true. In addition, there
are lots of ports that work "as is" on Linux. IOW, I never know what
purpose the `platforms` statement really serves.
FWIW we wouldn't be having this discussion if my proposition to support an
else clause for the platform syntax had been implemented already ;)