Master studies are a great learning tool if you know how to approach them. So long as they are not an exercise in creating a copy of another work, but rather a window into another artist's mind.
The reason drawing from life is encouraged is because you want to draw things how the human eye sees the world.
Drawing from a photo you are drawing how the eye sees what the camera sees.
It's not the same.
Ask yourself this:
- This area here has lots of fine detail, but over here it's all just loosely sketched in.
- Why would he do that?
- She used all these very de-saturated middle tones throughout, only right here there's this sudden swatch of intense orange.
- Why would she do that?
Is drawing from photos bad? Nope! Just not as good as from real life.
For the sake of argument let's say you're learning from photos. It depends on your goal. For example, it's best to practice anatomy from real live figure models. Well if you don't have access to those you're stuck with photos.
Life drawing classes are also good for seeing how other artists solve certain problems. "How can I portray a background in a studio setting simply but clearly?", that sort of thing. Now you could draw photos day in and out for days until you could copy them perfectly, but if you're just focusing on copying what you see, that's the skill you're most training, and you can expect to have learned very little about actual anatomy. And the ability to solve problems is then left out, because the photo doesn't have the same problems that you will encounter in a life drawing class.
But class isn't the only thing.
To really get the most out of them you have to know enough to be able to ask the right questions.
That doesn't mean you shouldn't do them; they can be fun and educational in other ways. But if you go too far down that road it can be hard to break away from, when you're trying to make your own work and start comparing it to the studies.
Which brings me to master studies. Master studies are excellent because it keeps you from spending massive amounts of time reinventing the wheel. If you copy them just to copy, much like the photos, all you'll train is your copying skills. If you approach them as an anatomy study, you'll learn anatomy much in the same way you would from photos and like a photo you will be copying the stylistic choices.
That means that there is a danger to doing that. But at the same time you can learn a lot about how to approach your own figure drawings by watching and copying how other artist's approach theirs.
A master has already solved all the drawing problems. Take those answers and keep them in mind while drawing from reference.
The study can be of anything from any artist. How the lighting is set up, how colors are used, composition etc. Producing an exact copy is not needed. You just want to find the master's answers to the problems you are having.
Mix it up!
Just so long as you're aware of what you're practicing, why, and any potential downfalls to it, you should be fine.