I'm aware of that. I'm interested in subnet banning more for denying people who try to evade a single ban. Subnet bans have proven effective for us in the past for users who try to evade by exploiting dynamic IP addresses.
For instance, we ban someone who was cheating using an aimbot or wallhack, etc. They are not happy with that, and decide to try to evade the ban by buying another copy of the game on an alternate account, and restarting their internet modem to generate a slightly different IP address. They could do this as many times as their bank account would allow if we don't have a way to block their typical IP range.
Of course they could always rent a proxy server after that, but if they have to rent a new proxy server every time we ban that proxy's IP/subnet, eventually they'll probably either loose interest or try to DDoS in anger.
Of course there is always the possibility of an innocent IP getting caught in the IP range, but probationary procedures can solve that for single IPs.
For instance, allowing a single application for exemption to be filed from any IP in the range once per week, then supervising those IPs regularly. This would effectively wage a battle of attrition against evaders in a given subnet range, while still allowing the occasional innocent player through.