I agree with Yzen Danek. In terms of an ultimate goal, especially in terms of researching a technology first or finishing a wonder first, generating Great People is a race. The nature of a race is that if you're not first then you lose. Thus every additional benefit, despite dimishing returns, becomes even more valuable.
Taking the "limit" so to say, of this scenario, let's say that one of the victory conditions is to generate a certain number of Great People. For example, in addition to space race, culture, etc.., the first person to generate 101 Great People wins the game. In this case, even if the "relative" benefit of [Philosophical + Pacifism] over just [Pacifism] were so tiny that only 1 extra Great Person would be generated for every 100 Great People normally generated, the "ultimate" benefit is worth inifinitely more because that extra Great Person wins you the game. Everyone else's efforts towards generating Great People are reduced to worthless. In other words, taking this limit, worth of [Philosophical + Pacifism] goes to infinity and worth of [Pacifism] goes to zero.
Similarly, every Combat promotion increases a unit's strength by 10%. With Combat1, the promotion confers 10% more strength. Combat2, though, will confer less relative strength. Convoluted combat math aside, is it not true that in the big picture, the average Combat2 unit stands to defeat the average Combat1 unit? IE, the Combat1 unit stands to die completely and the Combat2 unit stands to live and fight another day (while getting more experience).
In race situations, the relative benefit tends not to mean anything, so long as its positive.
Also, please explain something to me. Does the required number of GPP increase for everybody once *anybody* has made a Great Person, or is it based only on the number of Great People you *yourself* have generated? In other words, if by the time I generate 1 Great Person, 3 others have been born elsewhere as well, will my next Great Person cost 200 or 500 GPP? I never paid attention to this before.
