SITUATION AYou have a brush in your map that you want to act as a blue light.
SOLUTION AYou search through Karith's textures.
You find a blue light shader.
You use it.
SITUATION BYou have a brush in your map that you want to act as a light-emitting liquid.
SOLUTION BYou make your own shader.
You use it.
I hate to say it, Fish, because I think you're an intelligent person but, if you used SOLUTION B for SITUATION A, you're redundant and stupid. You slow your workflow and create that much extra (cloned) content by recreating the wheel
exactly as it is. There is simply no point in creating a shader if one already exists for what you need. Now, if someone needs an extremely obscure shader they will figure out how to make it. If they can't do that on their own they're not qualified to make maps, anyway. Whenever I need a custom shader (as I've been too lazy to learn the language because I don't map enough for it to be worth it) I check the internet. I'm capable of using a cheat sheet to construct shaders. Everyone in the world is. If they know that there is a way to create custom shaders, they're all set, and will learn whatever they need to learn whenever they need to learn it unless they're content with pumping out (less than) mediocre maps.
TL;DR:Face is saying that if an asset already exists, use it. Fish is saying that absolutely nothing in your map should be premade because that's lazy and stupid. Okay, so, that's a bit inflated, but the amount of drama here rivals the amount of drama surrounding my senior prom. Which is today. Seriously, kids, wrap it up.

You're both right. The choice between custom and premade is subjective to the situation. Sometimes custom is better, sometimes premade is better. It's simple as that. Go clean your tampons and stop posting.