A Druid (from 13 True Ways, page 38) gets +0 base AC and a -2 attack penalty for having a shield, going by the 13th age PDFs. There's an asterisk note that says:
* Warrior druid talent choices can change shield and one-handed weapon stats.
A Warrior Druid in particular can choose the following for a benefit:
You can use a shield without taking an attack penalty like other druids. (For the record, you only get the shield AC bonus while fighting in humanoid form.)
... which brings us to +0 base AC and a -0 attack penalty for having a shield! There appears to be nothing further directly affecting shields. At this point, it seems more worthwhile just sticking with a weapon and nothing else. (That's surprising, given there's another Warrior Druid benefit for using heavy one-handed melee weapons without penalty.)
Is there a particular point to a 13th Age Druid, or Warrior Druid, picking up a shield? Is the sole point to get benefits that might otherwise be attached to a Shield magic item?