"What's wrong with explicit opt-in consent *every* time they want to use your data, saying exactly what the use is?"
The problem with any system that provides patients with a choice about use of their data (opt-in or opt-out) is the danger that the set of data allowed to be used might not be a representative sample of the population, and so any analysis done using the data would be skewed. So, imagine that well-educated middle aged men with bad eyesight and poor social skills are the most likely to be sceptical of allowing data sharing. Your sample will be skewed against people who are well-educated, or are middle-aged, or are men, or have bad eyesight, or have poor social skills.
A system that provides all the data and which is trustworthy is the best system. The tragedy of the care.data farce is that government have behaved badly and so lost trust by trying to run it as a money making opportunity, rather than a public health care improvement or research opportunity.