My two pesetas on this question:

When I plane or scrape carefully (all of the following pertains only to woods  
without gross tearout propensities), the wood has almost a glasslike surface I  
look at it at a low angle to the light.   I don't get this when I sand, even
with   
220, 320, or 400.   Maybe if I were extreme enough to go to 1000+, though, as if
scarysharp(TM)ing the wood, but I don't.   And even from above, the wood looks  

smoother and glossier if planed or scraped than if sanded.

This is very important if I'm using a penetrating, non-surface finish (read:
oil),   because what I see, unfinished, is what I'll get, finished, as far as
the surface   is concerned.   No way a 220-sanded board will be glossy with an
oil finish, or   even a 400-sanded board.   Smooth, yes, but not glossy.   I
can't imagine even a   
1000-grit-sanded board being glossy with oil.   Wax can make up for some of the
non-glossiness of sanded oil finishes, but a planed board is inherently shiny
even   
with an oil finish.

It's still important if I'm using a low-build finish, such as a wiping varnish  
(e.g., Waterlox), an oil/varnish blend, or French Polish.   This will add some  
gloss, but the surface of the board still establishes the character of the  
finished board.   A non-fanatically sanded board (220-400) can be made glossy
and   smooth with a low-build finish, but it's glossy because of the finish, not
because   of the wood.   I frankly don't know if I could tell the difference,
depending on   the number of coats, but there's a difference to me that I
believe should be   detectible (at least with a limited number of coats),
perhaps as subtly as how the   figure of the wood may have more transparency and
depth when planed/scraped than   sanded.

If I'm using a high-build finish, such as varnish (poly or non-) or some lacquer
  or shellac finishes, all that will be seen is the finish, for the most part.  
I   don't spray, but a multicoated spray lacquer finish can be made as glossy,
smooth,   and glasslike as one wants.   There still may be some advantage to
having a finely   planed or scraped surface over a finely sanded one in this
case, but it seems to   me the difference would be subtle indeed.   Perhaps the
difference between a 200   and 400 sanding job becomes invisible with a good
built finish, and I'm willing to   imagine that the difference between a finely
sanded and a planed/scraped finish   would be hard to detect too.

Where the difference between sanding and planing/scraping is most pronounced in 
 favor of sanding is when the wood is to be stained (not dyed).   Sanding is  
necessary to create the ridges that hold pigment.   A mirror-finished board
won't   hold stain as well as a 220-sanded board.   The finish (high build)
supplies the   gloss and smoothness that sanding takes away.

After and during finishing, sanding and finer abrasives are entirely
appropriate,   because one will be abrading the finish, not the wood.   Any
built finish is   essentially a plastic and can be given an arbitrarily high
gloss by use of the   right grits (including pumice and rottenstone).   Since
finish doen't have a grain   structure, there's no danger of tearing grain at
this point, and one can use   whatever level of abrasives you want to reach the
desired polish.

Flamers, fire away!

-- Michael D. Sullivan, Bethesda, Md., USA 
-- Email: mds@a..., avogadro@w...