It has annoying downsides like the need to setup a complete "project" with build options and all -- goodbye "build system is enough".
This is exactly what I love about Geany - whatever sources you feed it with, it parses them without knowing how to build them - whatever build system is used. And most opensource projects use autoconf/automake and it's impossible to have a look at configure.ac all m4 and makefiles and know which way ti will be built this way (moreover it depends on what's installed in the system, the config flags given during configure etc.). So every project would have to be setup manually by providing all compiler flags, defines, dependencies, etc. That's undoable for anything bigger.
This doesn't mean I would be against e.g. the clang parser @codebrainz did but I'd really want to keep the current way of parsing preserved for people who want it for the reasons above (it could be disabled by a plugin if someone wishes to do something else).