I pretty sure it isn't illegal. I do know that if you want to maximize your DPS that having one and binding say frostbolt/shadoebolt/whatever to it will allow you to get maybe 1 extra cast during a boss fight.
Its a normal hardware mouse and no special-just-for-you build one. When you use 3 Monitors and play wow on 5760 x 1080 - you see much more than every normal player, because in wow the 2 monitors on the left and right are added and not streched. Its not cheating. The game itself has been created this way.
ILikeEggs wrote:The infinite mousewheel isn't really prone to abuse due to the game's GCD factor on nearly all spells. It is still entirely possible that it's illegal though.
I would want it to be illegal just because it adds unnecessary packets to the server, for almost no reward.