Personally, I think it comes down to personal choice....if a person responds to someone on their ignore list, it's because they chose to do so. I guess the functionality that you describe wouldn't hurt the board as it would only affect members who have users on ignore but my bet would be that such a functionality wouldn't prevent members from interacting with those on their ignore list because in the end, they do so because they choose to do so. But if you think it'll lessen the workload on moderators, then pretty much, it's a must do.