If you ask me, in what situation would you require the capability to shoot over 1500 images on one go on the Nikon D90? The answer - When you need to do both images and videos on your day shoot.
Frankly, the use of interchangeable lens on a DSLR coupled with HD video recording can almost mean the death of your current video camcorder but this would also mean the need for a bigger battery capacity.
According to Nikon, one minute of video recording equates to one percent of your battery life. Hence, if you are planning to shoot and take video for those holidays of yours, you jolly well pack the MB-D80 along. And if you seriously cannot locate a recharging power point, at least you can still use those run-off-the-mill AA batteries for your Nikon D90.
One word of caution while installing the battery grip though. The three battery pins within the chamber of the Nikon D90 may not be properly aligned from factory, meaning to say you need to manually adjust it in order for the battery in the MB-D80 to be properly recognized. This is very important and will save you countless trips to the Nikon service center.
That aside, I personally find that the whole camera is significantly more comfortable to be carried with the use of the MB-D80, and portrait shots are now composed so effortlessly. And if you choose to use one single battery, it will still work.
Lastly, kindly take advantage of the AE-L/AF-L button on the grip or you will just be wasting that functionality (and your money) entirely.