One of the things I've realized after working on SpiderMonkey for a few years is that simple solutions that accept a little bit of overhead are often faster than more complicated approaches that try to eke out every last cycle.

Math is fast. Stack space is rarely a key constraint. On the other hand, tracking typical argument counts for a function also requires overhead, and compiling multiple copies of a function is also not free. If a particular call site is hot enough that call overhead matters, it's probably hot enough that inlining is the correct answer.

As one additional data point: V8's new approach appears to be very similar to how SpiderMonkey's been implemented all along. Convergent evolution is generally a good sign.