Yes, they are welcome to build/test dApps all they like on EXP live. (The lower EXP price means they are much less expensive to actually run) -- The main use case as I see it is the community having the freedom to modify the project as we wish over time, and experiment with different paths/choices. Different block times, counts, rewards, funding design, difficulty settings, and so on. We are not locked in to the exact same choices that ETH makes.
Yes, the EXP price I thought also would be the main attraction
along with a live environment. As for those consensus variables that you
mention, it is generally not a good idea to change them without very
solid reasons.