Is there possibly a utility in at least using your own interrupt for system calls, so that you could implement a linux compatibility layer down the line? Linux syscalls are a little bit odd relative to what you would generally do in the modern era of tons of memory.
Is that not slower (requiring a memory store and a SUB, then a load and an ADD to pop) than just MOVing a register?
As a bonus, ptrace would allow you to host a slightly modified copy of your kernel in userspace, similar to User-Mode Linux. (Or you could modify a copy of User-Mode Linux to run on top of your kernel, as a heavier weight version of the syscall translation layer mentioned above.)
Other advantages of implementing ptrace include some debugging and sandboxing tools. Note, however, that if sandboxing a multithreaded process, if the sandbox intercepts a syscall where some of the arguments aren't in CPU regisetrs, the sandbox needs to pause all of the other threads in the sandboxed process, otherwise they may modify "safe" syscall arguments to be "unsafe" syscall arguments between when the sandbox inspects them and when the kernel actually processes them.