Draping vs. Drafting
... Nor did someone draping a garment have any concern for how the pieces looked when laid flat: an eighteenth-century garment looks very strange to modern eyes when it's taken apart to make a cutting pattern, and there was much more variation in the shapes of sleeve caps and the like. ...
Garment draping is much easier than flat-pattern drafting; you have the fabric right there and can poke and pull it until it's right, without really knowing any theory, where flat drafting is a sort of dead-reckoning process.
But your draping teacher has to be physically present, and flat patterns, being flat, can be put into a book.
http://roughsewing.home.comcast.net/~roughsewing/RUFFTEXT/RO...

Flat pattern methods take basic sloper patterns and alter them into more
sophisticated patterns. This is probably the most widely used method. The
basic principles are slashing and spreading. Some pattern makers use pivoting in addition to or instead of the spreading step. The books on flat pattern drafting show you how to cut lines on the original pattern and manipulate the pattern pieces to get a completely different pattern. One basic principle is how to move or eliminate darts in order to change the drape of the pattern and the design ease. Pivoting is particularly useful for moving darts from one part of a pattern to another.
http://www.faqs.org/faqs/crafts/textiles/books/part3/