I do agree that letting two cultures run parallel for a certain time, or aspects of a subculture for longer, will work well – it gives the employees of the acquired company something ‘old’ to hold one to, making the transition less stressful, or appearing less stressful.
Strong subcultures or values, though, should be targeted to be integrated at some point; thus even before going into M&A conversations the buyer should have a clear vision of which subcultures they want the target to ‘take over’, think of strong values like continuous improvement, a learning organization, respect for the individual etc.