Especially where in a D90, the grip does nothing extra but lets you insert 2 batteries. If you have time to spare (esp on a trip), why not get 2 or 3 batteries and leave them in your pocket or camera bag? I think it works better that way. How long does it takes you to swap out batteries? 10 seconds max right? Cannot spare that time meh?
Even for a D300, where using a grip with a D3 battery will boost the speed to 8FPS, I still cannot find reason to use one.
If you need a grip to make your setup look pro and make you feel confident, by all means, go ahead. I just think it is better to build up good confidence by learning how to handle your camera better, so when the moment arises, you can get that shot, and produce stunning images.