Growler,
I used to think along similar lines to your post. The during a recent visit to Cairns Tower & Approach, asked the question. The Approach people were able to demonstrate, in real time, how pop up traffic, either with or without a flight plan, can drastically alter the planned flow.
Oft times they try to squeeze every one in, rather than asking the bloke leaving Cooktown to wait 15+ minuted for the previous flow to pass through. Add to this VFR with no details, that presents at the zone boundary, and it's much easier to to understand why the late notice, as frustrating as it is, occurs.
Now what really gives on the sh!ts is requesting a visual approach at 150 to 200 nm, being cleared on a STAR that adds 30 odd nm, with altitude restrictions that keep one quite high, and then being cleared to shave 20 track miles off with only a moments notice. You'll never guess which northern port does this routinely.
