I think Windows Defender is useful, and I use it as part of My layered protection. Its very light on recsources, I never really know that its even on.
Its updates are fine with Me, as it is part of a layered defense, along with on demand scans with Malwarebytes and Superantispyware.
Maybe Im biased though, it did save Me from a nasty that crashed My other laptop hard, when debugging it and getting things back up and running and putting avast on it, windows defender found antivirus pro I believe and took care of it.
So, I let windows defender run and do its business, its not hurting anything.
Saty