ok, so that's actually one of, or the most fundamental requirement. The connection between user and 'network' HAS to have a fixed rate.
Assuming "fixed rate" means "always filled to said rate" not "fillable up to said rate"... then that makes every users node look nicely busy. And if the rate is the same for all users, then every user looks the same. However all nodes in the net need to be always filled to some rates. Otherwise adversary vampire can just watch the nodes end user is connected to, or perturb the users packet stream, or wait until user unluckily routes across quiet middle nodes, etc.
last but not least, you could apply the padding traffic to key pre-distribution or opportunistic protocol maintenance. e.g. distributing routing and node identity information. (the "directory")
If pad fill can be used to carry something, better than to waste it.