I originally skipped over the Viper III, until I lost my much nicer ship due to a stray shot in bounty hunting. That's when I bought a Viper III, because it lets me bounty hunt when I need to at 1/10th the rebuy of my fancier ships. Well, it's 1/5th now, because it's A-rated and tanked out, but it's still way cheaper to replace if I bite off more than I can chew.
I should mention that my approach is a bit different than yours, as I'm building different ships that specialize in different purposes, vs one ship that can do everything well. Granted, my Keelback carries lots of cargo
and can put pirates in the grave, but I've gone with a more defensive outfitting on that ship instead of offense. I've got some mean weapons on my DBX, but most of the modules are D rated for jump range, and I've got no extra hull enforcement. My Viper III is a fighter, period. I did put a 2 ton cargo slot in a spare slot, which I fill with some valuables as bait. Please pirates, interdict me

All the other slots are shields and boosters and lots of hull reinforcement (my V3 is more of a tank than many V4s out there) and other BH tools.
ps - someday when I'm rich and rebuy is no longer an issue, I'll try the V4 in a similar configuration. Rebuy cost is my #1 reason for picking V3 over V4 right now.