I have used both products for years, and have found that both of these products have matured over time.
I've found no significant issues in running both since ~Win7, albeit I'm certain there is redundancy, and find that both products still have strengths and weaknesses.
By running both products, I find very few occasions that I need to go looking for other cleaning utilities to keep my malware PC performance in check (via SpyBot), and most viruses away (via Defender).
Personally I don't get involved in this type of finger pointing - I really could care less who thinks they are right since both companies are making their customers uncomfortable, which will make them look for other solutions. What I do feel is important
is for each provider to take responsibility to provide adequate information on what is causing the phenomenon (this forum helps), and options on what to do about it.
In this case I feel both products are contributing to an easily solvable problem... Microsoft had no problem with S&D running prior to Win10, but now after Win10 upgrade the user community is instantly confused why they are getting Microsoft defender messages
that they never had before, and Microsoft's error messaging doesn't help the user understand what is causing the message, or what options to take to do something about it. I think it a bit ironic that the Defender message basically tells the user to go get
a different virus protection program.
In looking at the S&D options, I'd think that most users would want to have S&D "integration into the systems Windows Security Center" as that type of tight integration seems like the right choice, but that choice currently deactivates Defender.
It does make sense to me that if the S&D virus protection feature was installed that I would want to deactivate Defender since then I would be running 2 virus and malware protection programs, and that seems like 1 too many.