I'd say that this should be left to user-mode translators (eg, something like QEMU but for Windows/ReactOS). Most Windows applications are currently built only for IA-32, it's hard to find things compiled for other platforms (Win64 is gaining popularity, but still...).
Running ReactOS on a platform in which Windows as limited or no support already, such as SPARC or PowerPC, does indeed leave the user practically left in the dust in terms of compatibility. However, most users will likely be running ReactOS on x86 or AMD64, so in-kernel translations of binaries will just add bloat that most users will never need. It's my opinion that if any project is made to run non-native binaries (like what QEMU on Linux does), it really should be left in the user-space, as an optional thing to run, but does not add any bloat to the core system itself.