When i first began learning anatomy and facial structures and such i use to do quick drawings like this before. Then some professional artists on the site i was apart of was telling me the point to a study is to sit down and fully analyze your drawing and whatever aspect of it you are looking towards. So if you want to understand anatomy or porportions they reccomended sitting down creating a grid and systematically disecting each piece of the drawing to understand its actual placement and how everything really falls into place. (if your referencing from a photo).
But basically the point to studies is to sit down and fully analyze everything so they should be taken with just as much care maybe even more care then your finished drawing depending on your wanted outcome. If its just gesture drawing and understanding the overall aspect of it then yea quick drawings can suffice. But if you are trying to understand how a hand is REALLY drawn and how the bone structure and muscles and contract and create a piece, these can tend to take hours on hours of precise drawing.
Not to say what your doing is a bad thing. It was just something someone criticized me about. They say a quick drawing is better then nothing, but if you want to truly learn and understand the concepts you need to spend hours and hours on studies and really crack down to as realistic as you can possibly make it.
Some of these are better then what i can do because i stopped my practices with anatomy. After my projects i have in line right now i will pick them up again so im fairly certain everyone will see how terrible i am at it -_-.
TL;DR. Quick studies are good but you need longer more detailed ones with them too.