I've definitely built this sort of thing at least a few times. There comes a point where it definitely makes more sense to build this sort of functionality (or now use sendwithus!), i.e. when you get tired of hearing about changes to email templates from marketing! At the end of the day making it easier to test and optimize all the communications with your customers no matter how mundane the email is awesome. Just simple things like upsell messaging depending on the user is huge -- and not something a developer generally wants to or is capable of worrying about :)