I'd like to add that they also provide many useful features, like nice album views and bulk downloads. And I suspect those useful features, combined with the eye-friendly design, help make people much more likely to paste non-direct links.
Or is it because the founder of imgur told people he made imgur just to help Reddit users?
Before imgur, most links were direct.
The answer to this question should be interesting to entrepreneurs here: Is it a good strategy to create a business for a large social network under the pretense of public good? Is the sheer number of users more important than how monetizable they are and the likelihood they will reciprocate?
For example, since the pro account as no inactivity pruning, I bet you could setup a photography website that hosts tons of images, and use imgur as your primary image host to offset bandwidth use...