If there are crimes happening which require armed intervention, that is exactly a reason for having a professional security force. If the police does not show for such a crime, but you need to rely on security force, well good luck to you, and glad you have a backup on dial.
A security force is also going to cooperate with the police and perform surveillance. Not like they pull up in an armored van and exit with weapons drawn, because a neighbor called them for an illegal lemonade stand.
I don't think the killing of Ahmaud Arbery was by a neighborhood watch. Professionals would probably not have let it come that far. Same with Trayvon Martin: if a professional force had responded calmly, he may have been alive. So if a neighborhood watch is necessary (either for the feeling of protection or actual crime fighting) then a professional force would seem a lot better.