Problem with joining into a team that queued via the LFG tool that if you are doing daily heroic or daily normal the toon that joined NOT via the LFG tool will not get the end reward for completing the instance. So no valor for them from the daily heroic or no extra justice point in case of the normal instances.

Of course if you don't mind doing daily heroic/normal twice then this is not a problem because you can just join in with another toon who has already completed the daily.