I also gave some of the passengers dark lances, but those are just trueborn in a raider, I have many DLs left over from bits and many leftover passengers. I don't really find such easy alternative convincing, but that's just me. The problem with this is: what if you have other raiders with passengers, there would be pretty much no difference between raider & ravagers, besides it's possible to have 2 DL having from the side => embarked trueborn squad with 2 DLs.
It was a lot easier to convert the old raider into ravagers
The actual ravager is just such a cool model, I can't see anyone making a conversion from a raider that would be equally as good or even better.
The ravager has a lot of additional parts, not just the side gunners, it's worth it if you want some change amongst your raiders, besides it has 2 vehicles upgrades you don't find on the raiders.