It is all a matter of risk management
if you come from the south into Cairns you overfly Rocky, Mackay, Proserpine, Hamilton Island and Townsville at the very least.
so in reality you only need to consider the likelihood of an event occurring in the period between your PNR Townsville, which is likely to be the bottom of your descent/somewhere near the commencement of the ILS. Yes you will likely land in Townsville with only close to Fixed reserve but what are the chances, given the number of movements per year, of the runway becoming blocked in the time between the commencement of the ILS and touchdown? There often isn't another aircraft between you and the runway.
I reckon the risk is far greater for machines like the 380 and 747 where the number of airports capable of landing an aircraft of that size, particularly the 380, few and far between.
737's, A320's etc are unlikely to get into too much trouble I would think.
The problem, in my view, lies with people who think they must touch down with 2,3,4 tonnes even in an emergency.
And Framer - it would have cost a squillion to have that extra 30 minutes for every sector on the off chance - the cost of a diversion is far less than people think