The shield becomes a holy symbol, no argument there, but without the Warcaster feat the cleric can't perform somatic components with a weapon in the other hand. This is an intended effect, given the mention of shields as holy symbol. Now Warcaster is awkwardly worded, but it's pretty clear elsewhere that you can perform the somatic gestures as long as you have one hand free (shield hand isn't free/two-hander isn't free).