The origins of the internal external debate are essentially political. There is an excellent article to this effect in the Journal of Asian Martial Arts (if I have time I will look up the issue # and post it).
The first reference to internal and external arts in in a eulogy written about a ming loyalist general. Internal referred to the Chinese (Ming), and external refered to the conquering Manchus (Qing). Much later, there is reference to a martial arts political organization of the "Internal Schools", form mostly of the "sister styles" (tai chi, hsing I, ba gua). And on and on. Again, "internal" means something more like "native" in this context.
It seems to me that any exclusionary philosophy (such as separating internal from external) would be in violation of the fundamental philosophical principals from which the arts were born. Yin (In) and Yang (Yo). Mu. Etc.
Therefore, after many years of fumbling around this subject, I don't think the differentiation has any real meaning, although it may be used to designate a style as this or that for promotional or political purposes. The idea that internal styles use a more powerful or elusive kind of strength is attractive, but it conflicts with the fact that many "externalists" make the very same discoveries in their training.