Even if you're drawing from reference, doing a straight-ahead drawing is, without very good observational skills, going to end up with an inaccurate, disproportionate drawing. And you aren't just born with great observational skills. There's a reason why the current atelier method teaches students to
simplify lines and angles, measure, and slowly build up in complexity. These are schools led by professionals in their craft, and these methods are time-tested.
Many professionals in the industry - even those that do not rely on direct references for most of their works, still produce
sketchy drawings, and will often draw the same idea
many times, before committing to a final product.
You
can draw straight-ahead successfully, but its something to be reserved for when you already have good drawing skills, and you're not concerned with absolute accuracy, only to produce a basic drawing. Some artists like to do this a lot - some prefer to construct their sketches instead, and some like to play with lines and rhythms. At that point it comes down to preference and the artist's own style. Straight-ahead, like any other method of drawing, can still work as a good practice exercise, but it's not the be all and end all of drawing.
Very rarely will you ever come across an artist who is able to produce a complex, complete, full-scale piece from start to finish, without any physical reference, and without any sketching,
but it does happen. This guy's probably been drawing everything he's ever seen, almost all day, every day, for his entire life. I doubt most people who have posted in this thread get by one observational drawing per week.
Basically, for mere mortals like us, it is a lot easier to maintain correct proportion, form, volume, etc with construction and simplification. These are very important concepts if you want to draw with any degree of realism.
