PUR = photosynthetically usable radiation, a concept I haven't yet seen a good reason to dig into. Plants use light from the whole spectrum between about 400 and 700 nanometers wave length. Unlike what many people believe, they also use green light, just not as efficiently as they use blue and red light.
If it was hard to get enough light to grow our plants, we would want to get every advantage we could, and look for bulbs that wouldn't waste any light in the green wave lengths, which, if you think about it, would leave us with very drab looking plants. But, the bigger problem we now have is avoiding too much light, not getting enough light. So, why bother with PUR?
The main argument that Daniel suggested in a debate I've had over the years was that with higher PUR, we get more growth per watt.
The other issue really is what coloration and aesthetics a particularly color spectral curve imparts to the plants. This is far harder to quantify(thus matters little in terms of growth itself), but PUR and RGR (Relative growth rates) can be quantified. This is particularly true when you get down closer and closer to where P=R, or the light compensation point for lower energy input for PUR.