Waterpipe Shotgun

Since waterpipes are fairly cheap, having 2 in your hotbar can act as a double bar if you don't have one. This is good for fast combat.

Can one-shot scientists in the head with a slug.

The waterpipe shotgun is often used in "all or nothing" situations, since people don't care about risking it due to its cheap cost.. a good option to quickly try taking someone outside your base that has just killed you and taken your stuff (without risking another kit), and at the same time pretend you have mostly low tier gear in your base.
Also a weapon of choice for doorcampers, besides the DB. For doorcamping, unlike with the DB, the best strategy is to wait at the corner of the door (or a wall), have them exit their base and close the door, to then roll up behind them and shoot them in the back. The reason being is that the shot might be survivable, if they are standing far enough inside the base when opening the front door (who stands on the ledge while exiting?), and with a waterpipe you got really limited range and just a single shot, so they can easily close the door on you.
It is also often used as a troll or griefing weapon, again, due to its low cost. E.g killing people at outpost and similar acts..