We can only do classification (for now), and we need users to be able to provide enough labeling functions that are correlated well with the true labels. Text and some video tasks are examples of where it can work well, since it's possible to write heuristics over the primitives (words for text, presence of an off-the-shelf object detector firing).
There are other tasks where the right decision might be to go get a bunch of human-labeled examples -- an example I like to think of is how you would describe a chair, if you didn't have any object detectors lying around. That's a situation where a) it's hard for a human to describe what you're looking for, and b) you would potentially get more bang for your buck by just having an army of Turkers label a bunch of chairs for you, since everyone knows what a chair is.
There are other tasks where the right decision might be to go get a bunch of human-labeled examples -- an example I like to think of is how you would describe a chair, if you didn't have any object detectors lying around. That's a situation where a) it's hard for a human to describe what you're looking for, and b) you would potentially get more bang for your buck by just having an army of Turkers label a bunch of chairs for you, since everyone knows what a chair is.