- Fixes for compiler bugs; probably not that likely, but I'm used to knowing that on a free toolchain I can always try a newer version to see if it exhibits the same behavior. Being locked to an old version feels risky.

- New language dialects (i.e. C++11). It's annoying to not be able to use the same techniques you use elsewhere because your ASIC core vendor made your compiler decision for you.

- New compiler features. It's no fun to have some clever trick I use other places fail to build because it depends on a compiler warning or some other GCC-ism that's present in all my other environments.

I also like to write unit tests and other simple mockups that can build and run on a plain linux machine, so it's nice to keep the delta between the two compilers as small as possible.

Error messages, for one.