Rad Canyon's track is outdoors but they make a track inside the Jordan Equestrian Center several times during the Winter for racing. It's good fun, but the track gets demolished after every race weekend and has to be built the Friday evening of the race weekend. So, if you aren't able to ride during that weekend, there's no place to do so.
Their outdoor track is cool also, but again, there are fixed hours for practice (Thur pm I think) and they charge you for it.
Other BMX tracks I've been to are generally open to the public for practice. You just rolled in and rode if there wasn't a race going on. I'd gladly pay a little, but would want to have access to the track more than what's available now.
As for the OP, I think an indoor place would be awesome and well attended by several local riders. After all, the snow isn't always good and many (myself included) would rather ride bikes except for epic powder days. I agree with Norway that a dirt track (4x/BMX or DS or BOTH!) would be better than a bunch of hokey ladder bridges. I don't even think you would necessarily have to have many races, just the practice on a track is more than we have now. Not sure if there would be enough people to make it viable. . . but I hear real estate is cheap right now!