Question about weapons and armours.

[Goutlard]

I remember one guy once theorizing that the japanese didn't wear shields because they are a short people, and therefore, harder to hit with arrows, as the samurai warriors can stand closer to the ground.

Besides the obvious problems with this assumption, the greeks were one of the shortest people of Europe, and their favorite combat style consisted on using a large shield and a spear.

Regardless, if the japanese indeed fought mostly using the katana, not having second hand weapon (like a shield, or even a wakizashi or tanto) would be a disadvantage, as the blades are light and balanced enough to be easily wield with one hand. However, as it was discussed multiple times already, their main weapons were in fact polearms and bows, both of which require two hands to be fully effective.

That might have changed after the Tokugawa shogunate, but it wasn't like they were fighting any actual wars by then, and other than repressing rebels, combat was probably very ritualized, so practicality was the least of all concerns.

A few points on your first part ;
The Katana was not a Samurai's main weapon. This role was mostly that of the Yari, the Yumi. The swords (Daito or Shoto) being drawn if disarmed, or drawn in close quarters, since you'd need to be quite foolish to try to swordfight with a Yumi ! Or to grapple with a spear, and so on.

The reason why the Katana is that often portrayed or seen alongside Samurai is that a spear was quite annoying to carry in everyday life, whilst a sword was most easy. The Katana stood as a symbol of the Samurai, it became more and more important after the reunification of Japan. But, never would a Samurai in his right mind go to a battlefield relying on the Katana alone if he could afford anything better !

They kind of had shields, in a way. Look at the O-Yoroi, and it's large shoulderplates named Sode.

It appears similar in a way to strap-shields, which is to link with the evolution of the Samurai. The earlier versions of them being horseback archers, this allowed them to retain a good protection whilst being able to use a bow proeficiently.

Over time, the sizes of the Sode became smaller, and smaller. This can be linked to two things ;
- Increase of the armor's quality ; the top-quality armors changing from Iron - Steel to hardened Steel, the weaker points covered in Kusari. In a fashion similar to the use of full plate reducing the size of shields, the sizes of the Sode decreased.
- Freedom of movement ; the large Sode were quite cumbersome. Not a problem if shooting arrows to an ennemy from afar, however it can be in close quarters. And, as the way Samurai fought changed, that changed too.

 

[Kissaki]

The reason why the Japanese abandoned the handheld shield is something of a conundrum, and there is no consensus as to why. There is probably not one single reason anyway, but likely a combination of different factors. That samurai were mounted archers at first may be part of the explanation, but it wouldn't explain why it was also abandoned by common foot soldiers. Neither the Parthians nor the Mongols abandoned their shields,  for example, and they relied heavily on mounted archers as well. Personally, I think the key must lie with the infantry, with mounted archery being a contributory factor.

However, the shield seems to never have disappeared completely, though its use must have been very rare. There are only two depictions of hand held shields in medieval Japanese artwork, both from 13th century scrolls - and then there is one or two from Edo period art. An example of the latter is a cavalryman defending with a square, buckler-sized shield in his left hand, which I found here (scroll down):

http://samuraiantiqueworld.proboards.com/thread/28/use-shields-japan

The notion some people have that the Japanese didn't use shields because it was "dishonourable" somehow, is certainly nonsense. Were that the case, they would not have worn protection at all, and they would certainly not have used those huge tate shields, which were used en masse.