The f/2.8 version would be the lens to use for say a night time highschool foot ball game or very low light wedding senario, think first dance, and trying to keep shutter speed up to freeze action.... That extra stop can mean all the difference. Say you're shooting iso1600 @ 1/100th @ f/4 and need a faster shutter speed but don't really want to up the iso to 3200 (think D300) that extra stop can make a big difference in that senario. Also in very low light, the f/2.8 lens should allow the camera to focus easier/quicker due to more light coming in..... So those that need that kind of speed from their glass will still buy the f/2.8 lens..... If you're not working in those senarios much, then all means, get the f4 version and save on weight and money.... If I had the funds to have both, I would. I just bought the f2.8 not too long ago (just before the f/4 version was announced) and don't mined the weight when I need it.... I still have my 70-300 VR for day light shots if needed, granted not the same IQ as either 70-200's though.... Just my thoughts and examples for you.