In my opinion, it all depends on how critical you are with your sound. It's not like these companies push out duds, what happens a majority of the time is that one or two of the cymbals may not quite match the others in terms of sound. If you pick each cymbal individually, you're able to pick and choose as you wish. Say maybe a 16" Medium thin crash sounds a little high pitched compared to the others ion the pack, I would try a 16" Thin crash just to see if they sound a bit better when I'm choosing individually. There is a hugh tolerance factor when it comes to manufacturing cymbals, particularly from Zildjian and Sabian. In other words, pick five cymbals with the exact same model and size and good chances are that they're all going to sound a bit different. This is why I say about being critical with their sound. No question about it that you will save money buying the box sets and you might be able to re-sell the cymbals that you don't care for, but you will also loose money doing this. I've hand picked every cymbal I own, sometimes going to two or three well stocked shops to find exactly what I want.
Dennis