It's all about Marcus and Goris. This is the reason I've NEVER done a playthrough with either of them
They seem so awesome it would just feel like such a cheat to bring them along for the ride
Marcus is arguably weak in the endgame due to his inherent armour becoming redundant, but that's pretty much fixed with Killap's RP. Goris is an amazing NPC with sickening melee damage, and inherent armour similar to combat armour, whether it gets better as he levels up, I have no idea.
But I can see where you're coming from with Cassidy, he's a great out of the box small guns expert, and can pretty much rain on anyone early on provided he has the weapons. Don't give him melee though, he gets no strength bonuses. Small guns is where it's at. Not being able to give him chems can be considered a disadvantage, but I never used to get my NPCs high