What's not to understand? Sure it's illegal to have, but what's that going to matter when SHTF? I'd imagine it would come in handy if you would ever need to shoot.
the problem here is that "you imagine" not "you've done".
consider for a moment that if you get caught with that little item, it's gonna be ten years, and $10,000 not counting legal costs. and when you're found guilty, you'll be a convicted felon and not able to legally own guns any more.
a lotta risk for the chance that it may come in handy in the event you need it. the last MAJOR events that i can think of in Kali would be SF on '06, Kern in '52, San Fernando in '71, Loma Prieta in '89 and Northridge in '94
so that's somewhere between 5 and 46 years between major events. probably not good enough odds to warrant the risk of having it in a BOB.
if you're got something that your state doesn't allow. keep it stashed securely. better yet, just move someplace where you can legally have the goodies that you can't have in Kali!
now then, if you lived someplace where a suppressor was legal, i still wouldn't sugest it for BOB use. they're long and get hot quickly. kinda hard to have one on a gun in an IWB holster, would be VERY slow to draw from concealment.
on a long gun or SBR, no problem. if you can afford it and do it legally, no argument against them for defending the home.
and if you're on the move and things are sketchy... other people may be shooting without them. people in general tend to move AWAY from gunfire, or at least not move towards it. so if you're moving from wherever you are when things happen to wherever you're going... i'm not gonna be to concerned about a litle noise.
completely different scenario if you're hunting or something. probably not a good idea to me making noise then.