Wow this is exactly the strategy I've played. I take it to the extreme and put literally all my soldiers on one territory.

The ideal situation is when an opponent ends their turn in a weak spot (likely if everyone is trying to secure territory) with 3-5 cards in hand and you have 5 in hand. Then you can cash in, eliminate them and end your turn with 6+ cards in hand in which case you get to cash in on the end of your turn (sort of a hack. I think the idea is the game designer didn't want it to be possible to start a turn by cashing in twice) and then you can actually secure your new territory.

I think people who try to claim a continent outside if Australia haven't really done the math on how long they need to hold the continent for it to pay off.