There are a lot more crazies than criminals amongst that population; it’s not like anything is going to be worse than what they are doing now.
Sometimes sending a person trained to think in terms of ‘good guys’ and ‘bad guys’ might not be the best solution when the person in question is having a mental health crisis.
I think most people here would not dispute that. But as others have said:
1. It can be extremely dangerous for the social worker, and
2. How soon can they get to the "patient?"
Ideally you would send both a social worker and cop, immediately, to each situation. But that's not very realistic.