Another reason to have a second compiler is for safety-critical applications. In the assessment of safety-critical tools if something like a compiler can have a second redundant version then each one of them can be certified to a lower criticality level since they'll crosscheck each other. When a tool is single-sourced the level of qualification goes up quite significantly.

rustc (via Ferrocene) is already being qualified, and form what I hear it’s been fairly easy to do so, for various reasons.