What is the cost to the project of maintaining their own JIT optimizing compiler? I presume, knowing nothing of LLVM other than what I have read, that it will continue to improve in performance with or without Mono or any other single platform on top of it. Let's presume that the Mono developers switch to LLVM as their sole target platform. What else could they spend their time on if they don't have to worry about a JIT?
Note that I'm not necessarily asking these questions rhetorically -- I don't know a lot about this technology stack and want to validate my core assumptions.