Why is GTIN exemption bad?

Does having a UPC (GTIN) have any benefit if I use the FNSKU?

If I get brand exemption (which doesn’t seem too hard to do?), and then just use the FNSKU on my packaging, is there any reason to have a UPC? If not, does that mean people pay for a UPC just so they can get approved?

I see some people saying to avoid GTIN exemption at all costs, while some people say it is fine. I will be applying for a trademark and brand registry before listing my items.