Out-of-tree targets for LLVM are generally a bad idea given how fast LLVM is changed. Every change could break the out-of-tree target, and being out-of-tree, there's no way the upstream developers could notice that.
The other side of this approach is that when the mainline developers do not have much love to a particular platform, it is dropped altogether with little to no remorse - see the fate of Microblaze support, or even the good ol' C backend.
I wish we had a more stable API (maybe with a transformation layer) for the backends that may not probably need much of the new features. If LLVM had a maintained transformation layer from the "current" SelectionDAG to a stable one, it would have helped a lot.
