The reason for this is not so much technological as it is human. Agencies still build walls around their communications operations. Sure, there are some mutual aid channels that are pre-established for coordination between agencies, but the underlying issue of a lack of trust or desire to have total control keeps people from talking to one another in a crisis situation.
I would have to say that public safety agencies do need the cellular radio spectrum for their dedicated use. With the advent of social media and spectrum hogging video they need their own network if communications are going to be unhindered by congestion and trying to share the airwaves with the general public.
If that spectrum is provided, it will be an opportunity to do it right and learn to share with other public agencies.