Yes, I agree. Only drawback is that in such case the UART_ERR message would have to be dynamically generated, according to how many ports are actually used. Or do you think "hardcoding" all 4 UART ports into one message is ok? 
Something like:
<message name="UART_ERRORS" id="208">
   <field name="ORE_UART1" type="int32" unit="errs"/>
   <field name="ORE_UART2" type="int32" unit="errs"/> 
...