Not wanting to use it for portraits but for indoor low-light such as shows or rides, ie... Pirates or Alice in Wonderland or other places that require a low-light fast lens. I'm just not sure if a 30mm would be wide enough to get the picture. Take Spaceship Earth for instance, would you be able to capture all of it with a 30mm lens without having to back up so far that others would come between you and the shot?
Getting all of Spaceship Earth into a shot would indeed be a good use for an UWA lens. It's just not a lens you would leave on your camera or use very often.
For the dark rides, the "best" lens for a crop body, may be the Sigma 30mm 1.4. I don't know of any fast primes that really go a lot wider. And that should be wide enough for shots on the ride. (and currently on sale for under $300!)
While I understand all the imperfections of the 50mm, in terms of cost-benefit analysis, I love the 50mm as it is a fraction of the price of 30mm primes.
Anyway... so go the issues of building a lens collection. There are lots of situations that are best handled by a very specific lens, but such a lens may not be used especially often.
So it's really balancing the price of the lens, with how often you would really use it, and to what degree the needs overlap other lenses in your collection.
I don't own an UWA lens --- My widest is 17mm on the Tamron 17-50. A true UWA is on my "wish list" --- But personally, for me, for the way I shoot, it's been my lowest priority.
My lenses, from most used to least used are probably:
Tamron 17-50 -- my kit replacement
Minolta 50mm 2.8 macro (I just love the sharpness and macro ability of the lens)
Tamron 70-300 usd (standby for outdoor telephoto)
Minolta 50mm 1.7 -- indoor low light, occasional outdoor portraits
Minolta 35-105 -- Has a horrible minimum focusing distance making it useless indoors, but it's a phenomenal outdoor portrait lens. And it's a nice outdoor focal length range.
Minolta 70-210 f4 beercan -- Was my telephoto until I got the Tamron, still useful if I need the extra f-stop, and also produces some great telephoto portraits, but it has become my least used lens.
Sony 18-70 -- Sits on a shelf.
My "wish list" would be a 30-35 1.4, and UWA (maybe the Tokina 11-16 or Tamron 11-18), and maybe the Rokinon 8mm fisheye.