Depending on the pot (not sure which type you see) but taller pots are often used for cascade and other styles that travel far beyond the boundries of the pot itself (as a mean to add weight so it won't tip.

You also see them used sometimes in Penjing (the original bonsai)

Here is a link that might be useful: wiki

Sarah Raynor, a noted quality bonsai potter, mentioned to me that she has recently been doing more in porcelain than during something of a recent lull. I have several of her pots in that medium.

http://www.redwing.net/~daalms/index.html

Al

Thanks for the Link Al.
She really does wonderful work.
Paul

.... and she's a nice person, as well. I've see her pots holding some of the world's nicest trees.

Al