Slug throwers are inferior to blasters in most regards, adding the complexity of bullet ballistics that blasters don't deal with. But specialized units, improvised guerilla forces, and mercenaries make use of slug throwing weapons so there is a place for them in the Star Wars universe.
While most slug throwers cannot pierce a Stormtrooper's armor, they can absolutely annihilate Jedi as melted slugs can create deadly shrapnel, and can't be reflected back at the shooter like a blaster bolt could.
There have been a few situations where it seems feasible slug throwers were issued as standard equipment to standing armies, such as when Order 66 was carried out by the Grand Army of the Republic. I have not seen any such accounts but it's big universe far, far away and there may be examples I haven't heard of.
Were slug throwers ever made standard issue equipment of a standing army?