Bridge has roughly two personalities:
- switch-like entity which passes frames between member ports
- interface, which allows ROS to interact with traffic passing the switch-like entity. This one is implicitly created with every bridge, some of configuration in /interface bridge is actually about interface personality (properties are named same or similar to those on traditional interfaces, e.g. pvid, frame-types, …).
More complete explanation by @sindy here: http://forum.mikrotik.com/t/routeros-bridge-mysteries-explained/147832/1
So when you want to have bridge interface set up as access (or hybrid) port to bridge switch-like entity, it needs PVID set. If you don’t need interaction with bridge for a particular VLAN ID, you don’t make bridge interface member of that VLAN on bridge switch-like at all. If you construct a vlan interface on top of bridge interface, you have to configure bridge interface as tagged member that VLAN on bridge switch-like.
It does sound a tad complicated, but when you wrap your head around the concept, the pieces fall into their places naturally.