Yes, indeed, it is the same for maps. But you know that, for me, maps are just another graphic representation ;-)
When I was speaking about sketching a map, I was referring to a really rough sketch, taking just a few minutes, or even one sketched directly with the drawing tools (I have been sketching maps and tokens on the spot without problem).
A sketch for a drawing, on the other hand, can be a quite elaborate feature, sketching being also a style, not only a simple drawing. Yours are quite well done, and I don't think they could be made in a few minutes. Meaning quite a lot of work beforehand.
For me, anyway, even when I am just directly sketching in inks, without penciling <a href="http://cghub.com/images/view/335893/" rel="nofollow">http://cghub.com/images/view/335893/</a> ,for exemple, it takes much too long to work on the spot. Not to speak about some tweakings in Photoshop.
But, I suppose that preparing beforehand parts that are going to appear later in the adventure (objects, characters, locations...) could be quite useful because it would be possible to combine them when needed, using the map and layers structure that is normally used for mapping (the exemple given by Pleinair Allaprima at the beginning of this thread is probably a good exemple of this kind of use).
I am really missing the possibility to add a portrait to a token in Roll20. That would be so useful, even for non mapping displays where a token would be any part you place on the screen.
But still it means that you have to prepare beforehand what is going to appear in the next session. I think that it should be a good way to make a better presentation for a game, but I don't think that you can be sure to cover everything that is going to appear in your session. Particularly if you want to keep your players free about their actions.
Now, it would work particularly well for asynchronous games, giving the GM more time to react to players actions... I'll have to think more about it, because it is one use I have in mind for Roll20.