Welding patch panels in is *a* way to fix holes. For high-budget restorations, I would even agree that it is the preferred way.
That said, there are a lot of other options that are just fine for everyday repair and personal use. Many people don't have the equipement or skills to weld in new panels but are perfectly capable of patching holes with various forms of epoxy. I have 25+ year old JB Weld floor repairs that are holding up just fine.
As long as you get a good bond, fiberglass can be used as a structual member without any issues. It is regularly used for structures in marine environments where metals will corrode.
