Except that I could fail Titan heart phase over and over again. Without a parser, I'm not going to be able to see who's playing sub-par and causing the group to fail. You can say that the group is only as good as its weakest member, but how fair is it to then deny the group one of the primary tools they can use to improve themselves?
As far as needing it for AK. . .I've seen some pretty poor dps in there. Fights where we wiped to the first boss because DPS couldn't kill the golems fast enough to even really damage the boss itself. Fights where the Wall just pushes us inexorably toward the back of the room. Fights where we finish as a fifth Catastrophe is being cast. These are all times that I would've loved to be able to tell the DPS what they were doing wrong, but it's somewhat beyond my ability to catch everything that my teammates are doing while simultaneously running all over the arena contending with mechanics.
And honestly, the idea that people don't need parsers purely because they're still on easy content seems odd to me. The easy content should be building you toward the harder content, in the same way that leveling up builds you to the level 50 content. Letting people get by with lousy rotations and having no tool to correct them with just means that when they get to difficult content they have to forget everything they know about their class and completely relearn it. I don't know about you, but I'd rather learn BLM in WP where I won't often have to move than learn it while also trying to learn Titan Extreme. . .