You don't necessarily, but you would then have to have a very good to great OC. And if they are successful a team needing a new HC will posch your OC and you'll have to find another great OC and the cycle continues. If you have a HC that was a great OC , even if he hires a great DC teams are less likely to poach him to be their next HC. Or you can hire a motivator type like a Campbell or a Rizzi and then you'll need 2 great coordinators and both will be in jeopardy of being poached like the Lions will be next season