Well, considering a lot of wild/feral cat problems begin with domestic cats, here are some articles about feral cat damage:
http://www.australianwildlife.org/media/27964/AWC-Wildlife-Matters-Summer-2012-2013.pdf
Also these:
https://www.ehp.qld.gov.au/wildlife/threats/cat.html "Domestic cats also threaten wildlife. It is estimated that cats kill 3.8 million native Australian animals each year"
http://www.wildcare.org.au/Documents/enjoying_both_your_cat_and_wildlife.pdf
While you'll probably pick these links apart anyway, and of course I admit there is no 100% solid proof that this damage is being caused solely by cats, I'd much rather enjoy my pet without the threat of destroying my natural wildlife. Australia has a huge problem with feral cats, especially in my state (South Australia) and I'm not going to add to that.
Also the fact my cat has Calicivirus, and allowing him to roam outdoors would infect other people's cats and I'm not an irresponsible jerk, so. I have many reasons for keeping him inside.