It prevents people that want in on the Odin/Behemoth battles from doing so and it prevents people from entering a zone to complete a quest objective.
Zones like Coerthas and Mor Dhona are always packed with players and those zones are mandatory for story quest objectives and many dungeon unlocks. If you can't get in, you can't progress.
For Odin/Behemoth, I would be very annoyed if I couldn't get in because I want to collect the Odin and Behemoth gear and I want the barding and to do that I need the currency they drop. Now if they spawned more frequently or the currency and items could be traded it would be less of an issue (but cheapens the experience)
Though I do not like it, channels are the better option as opposed to being screwed entirely.
SE should encourage, not discourage more people playing. To not be prepared for the population we have, did Yoshi and the rest of the team think that little of their game's potential?