In the past, I have trained as often as 6 days a week, twice a day. For a long while I trained 5 days a week, then cut it down to 4 days a week. Before all my injuries I was training three times a week, and you know what? Everytime I dropped volume and rested more, I broke plateaus and added new size. I believe there is something to be said for periodization to the effect of upping volume real high for about 3-4 weeks then dropping back down to a lower volume or HIT style for a couple months. When put into practice, this type of training is extremely valuable for bringing up a lagging bodypart, or just spurring new growth, but the decreasing volume/increase rest is where most of the growth occurs. In conclusion, I thoroughly support HIT style training, but I also support volume training when used correctly.