This is important news, if only because many important warnings only show up when compiling optimized. If you usually build unoptimized during development just so you can use the debugger without getting lost, and therefore miss those warnings, maybe you can change now.
(The announcement reminds me that it's been a long time since I had to debug a problem resulting from optimized code behaving differently from the unoptimized. Congratulations to a lot of somebodies, on both counts.)
Almost equally as valuable as the noted improvement would be an ability to compile as if maximally optimizing, and actually emitting all attendant warnings, but writing out unoptimized, maximally debuggable code.