This could be useful, but only if the 911 dispatchers have extra training to be able to tell the difference between a call about a crazy person who is far more likely to be a danger to himself than to others (send the social worker) and a person who is a danger to others (send a cop).
The little heralded death of Tony Timpa, who was never accused of a crime, who, in fact, called for police to come to prevent him from doing something crazy because he thought some drugs he took were bad, and who, like George Floyd, suffocated under a policeman's knee, wouldn't have happened if Dallas had a program like Seattle is instituting and had sent a social worker. (Didn't hear about him in the media? Well, Tony was white, so a cop killing him didn't matter to the biens pensants who decide what's news worthy of being printed or broadcast.)
Of course, better still would be giving all cops the kind of de-escalation training MPs get, rather than training them to regard everyone they meet as a threat (which is how they are now trained). (Just one way the police should be more like the military -- not being allowed to unionize, having rules of engagement as restrictive as those we give soliders in counter-insurgency theaters, having as severe of punishments for killing innocents by breaching the rules of engagement, and, if not as a matter of law, as a matter of custom, making dismissal for cause from a policing job as black a blot as a dishonorable discharge from the military, would also help.)
