It really depends on what rules you have there about testing cars for roadworthiness. Here in the UK there is a rigorous annual test and any hole virtually anywhere underneath is a failure.
But again, any hole in a flat bit of floor is pretty easy to fix properly. It doesn't cost much to get someone to weld on a sheet of metal. Holes in shaped sections like box sections or cills are trickier, but again are pretty cheap, at least here.
If there are no rules, you can make excellent repairs just using old sheet metal, blind rivits, and resin filler. Splodge on lots of thick underseal, and paint the inside, and it will be like new. It only becomes not worth repairing rusty bits if the value of other similar cars has fallen so low that it is cheaper to buy another car than fix your own. But from my reading of the state of the market on Brickboard, car prices have bottomed out and it would nearly always be worth fixing.