I think its fair to say that if Management are not given equity prior to a merger, then the organisation runs a serious risk of those managers leaving, and finding safer/better/higher-paid jobs elsewhere.
If they are not worried about their job security in a new org, that is good. However it is possible that this is their first time in this situaiton, and they may start to worry as they learn more about tit.
It is rare for management to retain motivated without the right comp package that smooths over their transition from old-new, and keeps them performing at the level required.