Always add -L${prefix}/lib, -I${prefix}/include?

Many ports have had to add -L{$prefix}/lib to the LDFLAGS and -I$ {prefix}/include to the CFLAGS. Wouldn't it be much easier if the MacPorts infrastructure would add that to the LDFLAGS and CFLAGS generally for all ports? Could that hurt anything? I would think the same should apply to CPPFLAGS and/or CXXFLAGS, not that I have yet understood why there are two variables for C++ code or what the difference is between them.