I think it makes sense to have Great Philosophers, given their historical importance.
Not sure what particular benefits they would add, but I'll trust the devs with this. Maybe a combination or diversity of science and/or culture bonuses, or some unique benefits (similar to great merchants providing unique luxury resources)
The problem is, "Philosopher" covers a wide range of effects, most of them already covered by other categories of Great People. The game has to deal not with Thinking ("Lover of Thinking" is what philosopher means, after all) but with What You Are Thinking About: the concrete effects of the thinking.
So, Confucius (Kong Fu Tse) was a philosopher, but his thinking also started a religion and provided the basis for the education and administrative system that ran China for centuries. Plato, the "Father of Western Philosophy", also resulted in Neo-Platonism, which is purely a religion. Aristotle was a "Philosopher" but because his Thinking was about the sciences he provided the basis for both European and Arabic science for most of the Medieval Era.
So, most "Philosopher" effects in the game fall into already-existent categories.
What I think is needed in the Great People system is one more 'dynamic' effect and three more categories of Great People.
The new Great People are/would be:
Great Theologian - a use for all those Great Prophet points after you have your one and only useful Great Prophet. Historically, people who had great influence on the development of religions, but didn't found new ones. Folks like Augustine of Hippo, Bodhidharma, Colm Cille (Columba), Adi Shankara, Francis of Assisi, John Wycliffe, etc.
Great Minister - a Great Person that changes how and/or How Well your Civilization does things - a Super Governor, if you will. People like Solon of Athens, Alcuin, Hasdai ibn Shaprut, Tlacaelel, Thomas Cromwell, Axel Oxenstierna, Cardinal Richelieu, John Law, Benjamin Franklin, etc. These can have either specific or general effects, but in some cases and instances would have more influence than the Leader of the Civ.
Great Felon - the Random Event among Great People, whose 'bonuses', if any, come with strings attached. This is a place where the game can throw sticks into the smooth spokes of your Civ Wheel and force you to deviate from your well-trodden path to victory. Historically, people like Hassan-i-Sabbah (Founder of the Cult of Assassins) , Guy Fawkes, Stenka Razin, William Kidd, Mikhail Bakunin, Charles Ponzi - they cover probably the widest range of 'possible effects' of any Great People, but they would definitely keep your game from ever becoming Boring!
The new Dynamic Factor is that some Great People are great in several categories. What I'd like to see, then, is the chance to select where they will apply their Genius.
Some examples:
Thomas More - Great Minister OR Great Theologian
Leonardo da Vinci - Great Engineer OR Great Artist
Sebastien Le Prestre de Vauban: Great General OR Great Engineer
John Law: Great Minister OR Great Felon
Benjamin Franklin: Great Minister OR Great Scientist
George Marshall: Great General OR Great Minister
Walt Disney: Great Artist (Film) OR Great Merchant (Theme Parks)