Good question, I can chime in and share my opinion.
As a job I shoot weddings, real estate, events, and portraits. The lenses I own are listed in my signature.
My most used lens at weddings and events is my 24-70 2.8 II. Second most used is my Sigma 70-200 2.8 OS. I have a few primes too and just purchased a Sigma 35 1.4 ART, but mostly for low-light receptions. I also use my 85 1.8 quite a lot for prep shots and during reception.
I know I'm giving up light and DoF by not using fast primes, but on the other hand I can compose my shots better, and shooting at 2.8 on FF still gives a nice shallow DoF, if not super artistic. By not having to crop as much as with a prime, my workload is lighter during post.
During events (receptions, dinners, company parties) I pretty much don't use my primes at all, only using the 24-70 + 70-200 combo. Sometimes a lot of things are happening and I can run back and forth in order to frame the perfect shot with prime.
When doing the family photos after the ceremony my 24-70 is a great tool since the size of the group is changing all the time and sometimes it's hard to step back. But, like you, I also have the 16-35 f/4 IS and that can handle wide groups if I run out of room with my 24-70. But with time usually on short supply, it's better if I don't have to switch lenses multiple times during those shoots.
So for what I do the 24-70 2.8 II is vital.