I can tell you from personal experience that the Rokinon/Vivitar/Samyang/Opteka? fisheye blows. I own it, I use it, I hate it, I wish I would have spent the money on a better fisheye. I bought it mostly for virtual tours, but use it from time to time at weddings, for portraits, artsy crap, etc. I use it because I refuse to let equipment go unused, I consider it a waste of money.
It's a terrible lens for all sorts of reasons. 1. It isn't sharp until at least f8, so you're going to need lots of light in order to use the thing effectively. 2. No autofocus plus small viewfinders equals out of focus images. Compensate by using the focusing distance guide if you're going to be doing any work where you'll be closer than ten feet. 3. The image distortion on the edges is horrible. I know fisheyes distort images, but the are supposed to bend the image, not make it look like you smeared the subject. 4. It really isn't an 8mm, it's closer to 9.5mm. Some of their other brands even claim that this lens is a 6mm. 5. Build quality is bad, but, it's a $300 lens so I'm not surprised. Go on ebay, buy the Sigma 8mm for the same price or the Peleng 8mm, which has slightly better image quality and can be picked up used for less money.
That's just my rant about that lens, it's terrible. But on the plus side, it's cheap(which is why I bought it).
Now, would I rather have the AW100 over a bad fisheye, no. I have a few point and shoots, I use it for snaps around the house and that's about it. If I want to do photography, I pull out my equipment.
Your d7000 is weather sealed, I think, so rain won't bother it. I'd take the 18-105 and the d7000 for family outings. Personally, I take my D300 and 28-70mm on family outings where we may want decent photos.
I don't think anybody on hear can answer the question of "will it gather dust" for you. That's a personal decision. As I said, I find myself forcing myself to use it. It's been fun to learn, though I still have a ways to go on my fisheye techniques. But for me, I don't have much use for it. It doesn't fit my style well. If I were you, I'd focus in developing a style and good technique before you start outfitting yourself with a myriad of lenses. Otherwise, you'll just be a guy with no style and lots of lenses that you really aren't all that great with (please don't take offense to that).
I know when teaching assistants, the worse thing I can do is give them the 18-200mm. It simply provides too much range and they never learn how to properly work any field of view well (individual people are shot at wide angles leaving distorted people in mass amounts of space, faces are cropped way too close, etc). I find that their skills improve faster when using a narrower range, say a 28-70mm or an 80-200mm
Since you didn't list a flash, have you thought about picking up a used sb800/900 for roughly the same price? I use mine daily. It's far more useful then any single lens.