I have family in the emergency services, I know through them what a poorly mounted sub can do....
Tales of rear seats being torn from their mounts due to heavy weight subs, subs that have simply torn themselves free from the enclosure due to being mounted with screws....One driver even remarked how he seen his XXX 15" sub disappear under his front passenger seat at the point of impact, then reappear in the front footwell a mangled mess seconds later, he was lucky....
Its fine bolting down the enclosure, but will the enclosure withstand rapid deceleration, or will the screws holding the sub ???
I've come across a few pre-fab enclosures that have burst at the seams under normal use, in the event of a high energy impact or rapid deceleration these would simply collapse....
Always bolt the enclosure down, always glue & screw your enclosures, always use T nuts to mount the sub....