BRM transactions type

As my understanding, we have READ_WRITE and READ_ONLY transactions type.

Questions:
1. READ_ONLY is meant to preserve the integrity of the object while I read it so that no one can modify it and change the value. However, it won't change the value either. You could equally open using READ_WRITE.
2. If I'm correct with 1), if READ_ONLY and READ_WRITE transactions being opened at the same time and access to the same object, can the READ_WRITE still commit? And how the READ_ONLY transaction knows the object being commited by READ_WRITE transaction?
3. Would you be able to tell me any disadvantage if using READ_WRITE transaction only in order to access BRM object?