This has been discussed in several threads, and it warrants a poll I feel. The reasons for adding it is listed below, feel free to leave a comment as well.
1. Gives us a dungeon difficulty above todays heroic dungeons.
2. Alternative end game for players of a highers skill who don't have time for raiding.
3. Keeps 5-mans more relevant.
4. Easy to implement as it is mostly tuning.
5. Incentive for players of a lower difficulty level to improve.
6. Bigger pool of players for Mythic raiding guilds to recruit from.
1. Difficult to see how this can handle random queuing.
2. Many players grinding LFR today might choose this option over LFR.
3. There will be QQ and cries for nerfs if the mainstream crowd starts attempting these.
These are just some of the reasons for/against, I am sure there are hundreds of angles to look at this from.