As someone who has ported racket's for loops to guile scheme (or at least, re-implemented them without looking at the source of the original racket macros), a JIT has several benefits. There are some things that you just can't know at compile time, and hot paths/inline caches solve a lot of those things.
for example (for/list ([a (in-range a b c)]) (+ a 100)). Since in-range has to check when to end the loop, it has to know whether the loop goes in negative or positive direction (ie: to check the loop using < or >). If the values of a b c isn't known at compile time, the comparator has to be checked for each loop. This can potentially be very expensive, especially in an inner loop.
There are loads of optimizations that can be done much more efficiently/easily by a JIT. Store sinking, store/load forwarding, array bounds check elimination, value-range propagation and hyperblock scheduling to name a few.
LuaJIT is still the fastest dynamic language out there, but the complexity of the codebase surely makes you wish that wasn't the case.
