I think it really depends what role the previous leader played in the integration and in the development of the future vision and culture.
If the person is very engaged, supportive, interested in sharing the knowledge and a fit for the future culture I think it is really beneficial for the new merged company (process knowledge, customer knowledge, etc.).
If you experience already in the due diligence phase or start of the integration that the person struggles with the change it might be better not to further work with this person. However, if this is the decision I think it is key that the separation will be handled very professional including some knowledge transfer. If you do like this you demonstrate to all other employees that you care and you are not just fire the person