Being able to brace ones core is extremely important yes, and antiflexion/extension/rotation/lateral flexion and so forth is something that everyone should be working on. That being said, there has been a pretty dramatic shift in the last few years away from any movement in the spine. Neutral spine is not the end all be all that it is made out to be.
A lot of athletic abilities require a certain spine mobility and you need to work any synovial joint, which includes the facet joints, to have a healthy body. It's meant to move. Also the discus needs dynamic exercise to be nourished.
It's a balance.
I do agree that situps are pretty terrible though, especially for people with lower back pain, which is pretty much everyone at some point.
On topic, upper body strenght is secondary to technique and leverage in the clinch, unless you are a weak individual. People often forget that you get a pretty strong clinch by practicing clinching against good clinchers. There are so many variables. Is the other person taller than you? How well do they manipulate their weight? Where is their center of mass?
I have a hard time seeing how shoulder strenght and pec strength matters, but strong lats seems to help with the pull and strong legs/core with not getting moved around too much. Still, who are the best clinchers in the world? The Thais. How much weight training do the Thais do? Not much.
Not saying you shouldn't.