One issue that selecting files from a PLIST does not solve is conditional
use of dynamic libraries.

For instance, a port that creates a library linked against X11 and uses it
would be thoroughly hard to convert to firewall use that way.

Some simple cases could probably find out about X11 existence through dlopen,
and do the right thing, but this is hard to do. In fact, the only famous
program I know which does this kind of lazy dependency discovery is the Gimp.

In the few ports the OpenBSD team added FLAVORS too, there's a large 
proportion of no_x11 ports, precisely for firewall-like purposes and
other similar needs.
-- 
	Marc Espie		
|anime, sf, juggling, unicycle, acrobatics, comics...
|AmigaOS, OpenBSD, C++, perl, Icon, PostScript...
| `real programmers don't die, they just get out of beta'