Oh I see what you mean now. Yes you are correct, you would have to activate the input AO checkbox of the Base Material node and then plug your generated AO map in there. Otherwise the Base Material node alone does not calculate AO by itself. A couple of extra steps.
You're right, in this scenario it would be nice to get the AO generated from this node alone. On the other hand it might make the node more complex than it needs to be. I've never had the need for it to work this way, I guess our workflows are different. I think a lot of people simply see the basematerial node as a way to compile all your graphs and all of your channels in one neat package, in this case it would not be a node used to MAKE stuff.
But yeah I can see what you mean.