Andrew,
terracotta, used to be the preferred container in the older days before stoneware
and later plastic.
Probably, weight, breakage, firing costs etc.
Moved plastic up ---------- convenience.
No winter - no inconvenience - no frost ------ so we
still use earthenware pots for J.B.pines and you can get Chinese
pots that are side glazed, but porous bottomed for use with Serissa s.
and we do have one J.b.pine in such a pot.
If you read the older Bonsai books, the Japanese would take the plants
out of the pots and bury the tree's soil mass in the garden, with a shielding
of leaves. The pots being put away.
Or they had glass framed sheds for protection from frost.
So if you desire to use earthenware or porous clay containers, just read
up on the older practices.
Good Day
Anthony