Why is llvm considered unsuitable for implementing a JIT?

Question

Many dynamic languages implement (or want to implement) a JIT Compiler in order to speed up their execution times. Inevitably, someone from the peanut gallery asks why they

Answer 1

Update: as of 7/2014, LLVM has added a feature called "Patch Points", which are used to support Polymorphic Inline Caches in Safari's FTL JavaScript JIT. This covers exactly the use case complained about int Armin Rigo's comment in the original question.