Since the introduction of the solution framework in CRM 2011 (and now Dynamics365), I have not customized the default solution and find no value in doing so going forward.
Apart from their deployment and product shipping functionality, solutions provide a container where I can specifically look at what I am customizing and am working on as it pertains to my own entities. Whether it is an unmanaged solution in Production or Development (hopefully not in Production), at the very least I have a container that takes me two minutes to create that nicely encapsulates all the changes I have made.
With the Default Solution, I see the world, everything that is in it and everything that is a part of it. If I were to step away from this solution for a