The fact that $network is filled by the server is what makes it so useful for scripting, because it takes the server's actual ip out of the equation. This is especially important if mIRC starts scanning through a network's servers after an unsuccessful attempt. Also, if you didn't use mIRC's server list but simply decided to change servers or started using a bnc, you'd have to change your script each time you did so. That, again, makes it error prone. Yes, there are a few ways to maliciously take advantage of the server filling out $network, but it's really not a common attack vector (not to mention it barely gets you anything).