Lets see :
Shotgun slugs are non-penetratring projectiles . This does not mean that they have no armour piercing capability , and the way armour works in the B5 RPG (damage reduction) can make them pretty effective .
While most people here seems to forget is that B5 timeframe personal armours are much more advanced and protective than modern kevlar vests , but the most effective personal armour developed by humans , the reinforced flack armour has a DR of 5 and it has a weight of 20-22 lbs , while its Minbari equivalent (the absolutely best personel armour in existance) has a DR of 7 and a similar weight . But these are rare items , only used by special assault units , like the EA Hostage Rescue Teams , or some units of Minbari assault infantry , while the armour carried by a tipical cop/security officer gives a DR of 2 at best , and the armour employed by the EA GROPOS has a DR of 4 .
A 12 gauge shotgun slug does 2D10 points of damage , and tipically it will inflict around 9 DP , so against an armoured Security officer (DR2) it will do 5 DP (9-DR2 x 2) , enought to cause serious wounds to a 1st-2nd level character .
As for bring them to the game ... well , if you don´t like them , do not use them , but as matter of fact , projectile weapons have a few advantages over energy weapons , and this alone would be enought to convince anyone to employ them . Also , you should remember that a few projectile weapons appear on several of the books (a Minbari one included) , although I coincide with you in the fact that most races should have discarded them in favor of energy weapons (mostly to avoid ammo shortages) , and most advanced races (as the Minbari or Yolu) consider them barbaric as weapons of war (specially shotguns) . I am also covering some of this in my work .
PS.
I am also including a lot of "arcaic" weapons , as lever-action Winchester rifles or two-barreled shotguns (as well as rules about swan-off versions of these) for all the fans of Firefly (myself included) out there . I am also experiencing with rules for recoil and other niceties .