The whole point of security and good practices is to make harder for those arbitrary codes to be executed (or for whatever other flaws to be exploited, until they're fixed). Running a browser as root is not good practice. Doesn't mean that something bad will happen, but it's more or less line laying the cheese on the ground and expecting mice not to go after it. Even if you have none around, you just shouldn't risk it. At all. Unless you know exactly what you're doing.