Self-modifying code is just one example. The point is there is no way to positively identify all basic block entry points on x86 (especially due to its variable-length instructions). That means that there is always the possibility that the AoT didn't catch everything, and then your plug-in takes a novel uncached codepath at some point, the JIT fires, and you get a real-time constraint violation and a drop out.

Also, audio companies sometimes like to use "fun" DRM/copy protection systems.

Besides, audio apps themselves doing JIT would not be unheard of. For example, that'd be a very efficient way of implementing a modular synth. Those apps could JIT in a realtime-safe way; Rosetta can't (it doesn't have enough info).