If I understood it correctly, the purpose of this hash is just to shorten the method signature (name+types) to a 4 byte identifier. Unless a contract has thousa

If I understood it correctly, the purpose of this hash is just to shorten the method signature (name+types) to a 4 byte identifier. Unless a contract has thousands of methods, the chance of collision is very low (and the compiler could detect any collision, since it can see all methods). The "legitimate reason" for shortening the method signature is probably just to save space; bytes on distributed blockchain transactions are expensive.