I am just trying to gather some details before I start this project. Obviously the source code (I am using 2.4) was very intentional in its implementation to dis-allow having recurring and standard items  in the cart. Aside from being intentional, its a one liner in the code to change however I would like to know any of  the limitations encountered or reasoning for this constraint before I dive into a big project. I already have payment providers that will support this. What I am most concerned about is what it would do to refunding, the automatic recurring payments batch etc..., anything that lead the team to implement this constraint, any feedback from people who have done this, or any other thoughts and feelings regarding this. I absolutely have to do this (business demands it) so, DO NOT DO THIS, will not help me (please).