It's looked down upon because there's a lot of bad DJs who use them, more than DJs who use turntables, because they're cheaper and easier for people to get who are less serious about the artform.
It's more experienced DJs who look down upon it, because there's so many people jumping in and saying they're DJs after 2 months and taking gigs from people who have been honing their skills for years. It's not because the controller DJs are better, it's because they will do gigs for $50 a night, or do gigs for exposure. There's also a lot more people who call themselves DJs that really could care less about DJing, and don't know anything about the history, culture, art, etc... it should be pretty understandable why there's a stigma attached to it.
It's not that you can't be a good DJ with a controller under any circumstances. There are lots of people who use them who are good, and do innovative things with them, it's just rare. There are many cases when using a controller is completely sufficient and the best choice for you, but there are a lot of times when it's not sufficient at all, yet people use them because they're cheap and they don't care about being good at DJing. I'd say if you're playing house, electro, techno, trance, etc, a controller might be completely fine. If you're playing dubstep, or DnB, it depends what you're doing and what your style is, you might benefit from using something with more control. If you're spinning hip-hop, funk, breaks a controller won't suffice to be considered a good DJ. It all depends on your goals, and where you wanna go with it.
