I hope there aint gonna be ecosystem fragmentation
When there are a fews compilers with significant market share then developers are those who lose, lose that handy dev. experience of solid and consistent ecosystem
I hope it will be used only where necessary
I think this is more of an attempt of making Rust compilable on platforms not supported by LLVM, not an attempt to replace the existing compiler. The GCC frontend, for instance, does not implement the borrow checker, so you should only use it to compile Rust code you know is correct according to the official Rust compiler.
It seems like an odd idea to release a Rust compiler without a borrow checker. Isn't there a high risk that it will confuse users into thinking they have the usual Rust guarantees at run-time?
This can be very useful for platforms where rustc is not available due to LLVM not supporting them. Take a project, build it with rustc, and if it builds, you know it passes the borrow checker. It is then safe to build it for your actual target with gcc. This would limit the outcry when some software adds a dependency on Rust, like the python crypto thing of a few years back, or the Linux kernel.