I would not apply a cast to a limb suspected to be broken unless I had Xrays to confirm a fracture. If you are splinting a limb to reduce movement and reduce pain, we usually would like something easily removable to allow easy access to examine the said limb. To remove this cast, you would need a special saw.
Furthermore, after a fracture, there is the possibilty of further swelling to the limb. Swollen limbs enclosed in a rigid cast are a sure way of getting compartment syndrome, with potential risk to the circulation and viability of the entire limb in question. That's why, we put on backslabs or half casts over an fracture in the initial period and not a full cast.

HTH