However, I still believe that a determined outsider can get real benefit out of philosophy on his or her own. (Not the fullest benefit, but significant benefit.) I completely agree with you about cultural/historical/philosophical background, which is why in my answer I made a point of recommending not just titles but specific editions for classical works. Maybe we could call this the best of both worlds: good primary sources, filled out with enriching secondary material, all in one edition? (It does tend to make those editions more costly than some, and also heavier.)