Bandwidth wouldn't really be an issue, but yeah. ^^ It breaks the flow of the game/conversation you're participating in, that's why I've always understood it's banned.
Also, I guess this would be a good time to bring up, when someone double posts/breaks a rule Don't go off-topic informing them it's against the rules, just report the post without posting about it, and leave it to the mods.
An exception might be if they do an off-topic, but well intended post about, say, where to find Trainer Cards, you could point them in the right direction and then get back on the topic. But for the most part, let the mods do it.