Personally I think the immediate mode gui is not sufficiently simpler to implement over a retained mode gui. Their implementations are mostly identical, except that with an immediate mode gui you define your gui with more implementation. Why not take the additional small step to store the parameters of the gui in some structure? It's not that much additional work. From that point it's only another small step to be able to define your gui outside of code.

Personally I think the immediate mode GUI is not sufficiently beneficial to implement over a retained mode gui. Their implementations are mostly identical, except that with an immediate mode gui you define your gui with more implementation. Why not take the additional small step to store the parameters of the gui in some structure? It's not that much additional work. From that point it's only another small step to be able to define your gui outside of code.