There are lots of reasons NOT to arm some neighbors. My only point is that in order to have the OPTION later on to arm some level-headed but unarmed neighbors, you should buy the inexpensive “extra” guns now.
When TSHTF it will be too late, and your “option” will expire.
Thinking about it a bit myself, I can think of some situations, such as someone who pled guilty to the crime of misdeamor domestic violence years ago and is now legally prohibited from ever legally owning firearms (Domestic Violence Offender Gun Ban) - assuming that it was a one time thing and they don't have any problems now - who could be of great benefit if the situation really hit the fan (although I do believe that violation of the Lautenberg Amendment is a ten year felony, so it would have to be a definitely serious matter of life or death situation).