Well I don't know really. From what I understood, the only way you can "steal" another teams coach is if you offer him a promotion. So let's say my QB coach is 90 overall, but he gets an offer to be OC or HC in another team. I doubt he will wanna stay here.
That brings another possibility : coaches have traits. Traits rated on 5 stars like :
- Career (5 stars will wanna be HC asap)
- Loyalty (will stay on the same team longer)
- Respect (will bring FAs more easily)
- Development (will develop player quicker)
something like that... so a coach will become more important. right now its not really important to be honest