But there is a chance of things going horribly wrong! We lost all that by a one step success, and I'm afraid to think what would happen if we rolled a 10 or lower.
Would they really take that risk consistently and as often as they can?
Not the risk, no. If they could guarantee that specific trade every time, though, they would.
Mana matters because at the heart of things having more mana = more power in a really simple way. Sigils are quick to cast, powerful, and flexible in combat.
In terms of raw combat power Sigils >>>>>> Runes >>> Alchemy. Runes need to be pre-set which means they're incredibly powerful in the right circumstances, but it would be much simpler to learn a spell that let Ulos trigger a dust cloud than to forge bricks. But bricks are cheap and can be charged by his friends - Runes are good because of their utility (can't have a citywide hot water plumbing system with
sigils), and because they can last a long time (you can set up traps, alarm systems, create magical maps, etc, etc, etc)
Alchemy is good because of what it does outside combat, for the most part. More hours in a day? Better focus? Better hearing, sight, healing, etc? Alchemists who use their art in combat usually just buff up massively with Concoctions and use their superior speed/accuracy etc to get the edge in the Sigil game.
Usually the reason an Alchemist or a Runesmith can compete with a Sigil caster in combat is because those two disciplines lend themselves to being incredibly rich, and they can burn wealth (in the form of charged runic equipment or constant Concoction use) to be able to be equal with a Sigil caster.
Like, Ulos doesn't even get a class to learn how to fight, it's considered that useless for him. If he gets the drop on someone and kills them, sure, but if he ever faces down a Channeling 3 character in combat without intense preparation and foreknowledge his chances are really, really low. For most characters 5 points of mana is a nice bonus that will increase their power. For Ulos it might mean adding another spell into his combat (retreat) rotation, thereby increasing his chances of survival in a bad situation significantly.
It also means he can charge an additional Brick per day if need be, or charge the Rainbow Hammer for two deadly attacks
and have detection spells up. He's so mana-poor even a tiny amount can make a large difference.