I'd like the scenario better if I could afford to drop 3 to take the card. Best case scenario, I can fort to NA and Asia and have a shot at a 1-spot in Europe. Drop 3 to give me 6, so I can lose 2 and still have 4, and then fort in NA and Asia to keep my stacks there.
Once you get shut out of North America, it makes it hard to eliminate players there, and some people like to hit 3s right off to go for the card.
And if I drop 3 in Oz or SA, I usually load 'em up and go for the continent bonus. Doing so is risky, especially if the other players decide to take me out elsewhere, but with minimum loss of troops, it's worth it, I think, particularly from South America if you're strong on the borders - you can still get into North America and Africa from there. Oz is safer, but it takes longer to get somewhere once you break out.