OK. Let's be honest. With the significantly slower focus speed while in live view, who in the world would want to use live view in the first place on the Nikon D90? On an initial glance, perhaps only in two occasions - while taking HD video and attempting to preview how our customized WB, metering mode etc. is affecting the outcome of the subject we are about to take.
But again, the Live View may just be designed to fulfill this two situations. Some say, it has a third purpose - for manual focus shooting! Yes, you can also now see 100% of the image to be taken, not the 96% on the viewfinder. In addition, it may well serve as a platform for you to experiment or learn advanced techniques like EV compensation, White Balance on your subject.
In other words, there's a world new world with the use of the Live View function on the Nikon D90. Turn Live View on and try it for yourself today. But remember, your subjects better be a stationary object or a very patient creature to do this.