Hello peturdarri,
Agreed, it can be very useful for scenarios that don’t work with Source Generators. 
It is a good question. IL post processing (ILPP as we call it internally) was initially required internally for DOTS projects. It has helped to solve some problems but it has caused (and it is still to some extend) also significant performance regression during compilation time so that DOTS projects had to move a large part of their processing from ILPP to Roslyn Source Generators.
As we will migrate from our custom compilation pipeline to MSBuild, it might make this custom process less relevant, as, for example, you will be able to use easily projects like Fody to perform IL transforms.
That being said, we know that ILPP is not going away anytime soon, so we should revisit if we want to make this official. For sure, from what you are saying, we need to be very careful at not breaking it if many external projects are heavily relying on it!