As to why it couldn't interact with the running app others have given some examples but it couldn't even reliably call functions as they may be inlined or generics may only be compiled into their required type specific versions.
I do expect[0] Apple to open source Swift when it is finally released and it to be part of the LLVM project so you could embed whatever parts make sense in your own applications but it wouldn't be like an eval function as I understand it although you could probably use it to make your own playground system. Basically it would be an LLVM feature/library not a Swift language feature.
[0] Guess. I have no inside information and may end up disappointed.
There is a place for eval. Just not in a language which is going to be used on Grandma's devices.
Swift is certainly a language that relies on compiler optimizations. I don't want to change that.