If a body shop tells me that they can repair a 15" crack in my lower front spoiler (as good as new...) - should I believe them?

Or, will it always be structurally weak...?

 

15" is big.

However, body shops are just like any other business, some have perfected the craft better than others. If your guy has a track record of doing nice work, why not believe him? Ask him if he will warranty the work for the crack. If they say "good as new" then they should have no problem backing it up.