To talk about seiza we need to look on this from the historical and cultural point of view. Seiza was used and is still used by oriental people as a form of sitting as we use chairs. Why that position was chosen well we would have to dig deep in to the tradition. The main thing is that this is just a form of sitting. The traditional martial arts are using seiza because this is tradition and part of a cultural etiquette.
Now about good things about seiza and bad.
The position in fact was giving quicker reaction and ability to move from resting position to fight. Are we sitting in seiza at home or in work or in bar? Not really. Is it quicker to react from the chair. I think yes.
Sorry I ment to talk about good sides. This is a tradition and traditional systems use it to continue the tradition and I agree with that.
- long sitting in that position might cause.
- knee pain and in long run injury
- circulation problems
Did you ever felt pins and needles in your leg after long seiza? That what it is. The problem is that if you stop the circulation for a long time your body will gather toxins, and if there are enough of toxins and you release them quickly, you will be dead. I mean really dead.
"Suspension trauma" is common when people are subjected to car accidents and are compressed against stuff or fall from heights wearing harness and are suspended for more than 20 minutes. Some may argue that oriental people can sit for a long time. That is true for two reasons.
One is they practice from a young age and train their body to get used to that. You can do the same with time. Two, they use small pillows or wooden seats. So be careful when instructor is asking you to sit for 30 min. Regarding meditation, you can meditate in any position that is not restricting you to breath. I bet you can meditate longer when laying on your back than in seiza.