Well, the longer you let that go the more likely it is to fail under load. That would double or triple the expense to repair. As it is, treat it to stop further rusting and weld in a patch. If that panel crumples it will have to be cut out and a new section welded in.

As far as the danger of the situation goes, that depends on how fast you drive, where you drive, etc.

Personally, I would tend to that before anything else on the car.
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Mr. Shannon DeWolfe -- I've taken to using mister because my name misleads folks on the WWW. I am a 53 year old fat man. ;-)