I don't know if this has been posted here or not, but it is a good read.
http://www.frugalsquirrels.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=1;t=044387;p=1
This is a man that was living in a SHTF type of situation. It is not a definitive answer to SHTF questions but it does lend some insight.
In there he suggests having a pistol caliber carbine, and a suppressor if you can. I think any firearm can be a Serious tool. It all depends how you use it.
As he states in there, 99% of the time a pistol is what you will be relying on. That being said, when needing a long arm, having an UZI with a 16" bbl would not be my first choice (probably my AMD-65), but I can see some practical uses for a SBR 9mm with good sub-gun ammo or even a 9mm carbine with a suppressor. They are controllable, hi-cap weapons, that can get the job done within range.
They are only toys if you make them such IMO