Symbolism, Imagery, Allegory
You can't get much more literal with symbols than, well, symbols, which is exactly what these runes are. Plus magical powers.
Who Knew Calligraphy Could Be Deadly?
When you're in trouble, whether you have to pick a lock in a hurry or deal with a demon from the literal gates of Hell, what would you do? If your first answer is to draw a picture, than you'd totally survive in the world of City of Bones. Congrats?
Clary may not get to attend the Tisch summer art program, but she gets an art education of a different kind from Magnus Bane when he lets her read the Gramarye, a book filled with runes that takes most Shadowhunters years to learn.
The first rune he shows her "looked something like a winged spiral, until [Clary] tilted her head, and then it seemed like a staff wound around with vines" (13.42). We picture a caduceus, although we're not sure what the staff of Mercury has to do with runic education. Jace describes it as "the rune for understanding and remembrance. It opens your mind up to reading and recognizing the rest of the Marks," (13.49) so we guess it prepares Clary for future magic messages.
Then there are the Marks, or runes that are carved directly into the skin with the stele. Before battle, Jace, Alec, and Isabelle cover themselves in them, giving them extra powers, like better weapon handling. Most aren't permanent, though. They're kind of like temporary tattoos, except for the fact that they hurt like heck and leave your skin scarred. Wouldn't it be easier just to buy one of those cheesy tattoo sleeve t-shirts?
After all, the scars last a lifetime. Clary's mother has them on her back (either she's very flexible, or she had some help applying those Marks). Jace has them along his arms, and Valentine has a chest covered in pale crisscrossing lines. The Shadowhunters are all but condemned to "a life of scars and killing" (23.18). Lovely.
For Shadowhunters, it's all just a part of growing up. Jace says that "Most Shadowhunters get their first Marks at twelve" (17.58). And Alec succinctly says, "We mutilate ourselves" (5.13), to which we say T.M.I. But if non-Shadowhunters get too many Marks, they can die, or worse—become one of the Forsaken. Those are the brainwashed soldiers cursed to follow the orders of the one who Marked them.
So, this self-injuring process has its upsides and downsides. Still, the fact that cutting yourself is a rite of passage that eventually makes you stronger seems a little dangerous to us. Please note that these are trained professionals; do not attempt at home.