The ignore function is primarily for comms and invites, it won't have much impact on whether you see the person again or not -
- People on others ignore lists will not be favoured if a choice exists when match making players together
- If all players in a session have the same player ignored then that player will never be able to join that session
- If at least one player is neutral or friends with the ignored player above then they will be able to join that session providing there is no better suited players trying to get in at the same time when only one slot is available
So if you're in an instance of anything up to 31 players and someone on your ignore list happens along, they'll be in your instance unless someone else is trying to get in at exactly the same time, or all the 31 players have him on their ignore list, or another instance exists where he isn't on an ignore list.
Basically, it'll help you not meet the person, but probably not very much.