Much of it is just the xcom engine and the prevalence of armour in Xpiratez on enemies. The difference in 28 and 35 damage is pretty extreme when firing against a basic 20-25 armour on virtually everything. Fair enough, a 0-200% damage modifier does change things, but you can only really rely on averages. Its also why explosives seem so powerful (they hit bottom armour if they explode within 3 squares, often giving an extra 5-10 damage more than ballistic weapons would with that damage rating), especially with their 50-150% damage modifier. At least there's always some damage done.
Flamers (and everything with a blast radius) work similarly, so can be useful even with lowish damage. Eight shots a turn help too.
Bows, mêlée weapons and other strength/melee accuracy/throwing style weapons are a special case, just because they get better as your gals do. More damage, the better you get, is good. But as a bit of a ball-park figure, 30-35 damage minimum still counts, even on really quick head maths. Lots more is better though. With items with stat based damage+stun you can sort of go wild though.
I tend to look at 30-35 as the minimum cutoff damage for any weapon worth taking. The very minimum. Otherwise there's just too many targets that they're useless against. Plus they work really well against low armoured troops as a bonus.
This does consign a lot of weapons to black market scrap, but oh well. The extra money is usually handy anyway.
Cattle prods are fantastic for stuns, just due to how early you get them and having a base 70 stun damage. Remember the 0-200% modifier though, the fact that you can miss, and the fact that the enemy can dodge (I believe catgirls have a pretty high dodge chance, so regardless of mêlée accuracy, you'll miss a lot). Always go for rear attacks on anything where possible (hit and run, opening up that square for the next gal), and assume that it'll take 3-4 gals with prods to take down most things reliably. Yes, one can do it. But sometimes Xcom loves making you whiff shots, regardless of weapon damage stats. 2% damage is just as likely as 198%, and 30% dodge is amazingly good (bad) at making things miss in mêlée. That's on top of TU/stamina concerns. They're good weapons, but never take just one. There's better stun style weapons, but prods are cheap and powerful and accurate. They just need a lot of TUs and stamina to use. So bring a few, and never underestimate running in bikinis. Or tactical smoke grenade use.
Ablative armour would be a great solution though. Then every weapon would be useful with enough shots eventually.
tldr; Simply don't use anything with less than 30-35 damage (except flamers. Use lots of flamers. They're great. Lots of prods are good too. 4 is a minimum of each). Then things get done. You wouldn't take 1 laser rifle in vanilla and expect it to be a game-changer. The same applies to Xpiratez.
