I use both, but the '!bang' syntax is more convenient when you simply want to repeat a search using a different search engine, or want to use an engine you don't have a quicksearch for (you could of course add them, but sometimes it's just not worth the trouble for the few times you need it).

Because it won't work in every browser and making shortcuts the same across Firefox and Chrome is a hassle, plus you can never get up to thousands of shortcuts that DDG has.

Sometimes, I'm just experimenting. I had no idea that DDG had a Flickr bang because I never had the need to use Flickr before, but I just added !f to my query and I got the results I was looking for.

DDG is my new-tab page, so that helps. Plus, if the wiki page is lacking, I can just go back and do a normal search or a Google search or a Stack Overflow search just by changing a couple characters at the end.