I agree it also leads to click box healing addons, mods that tell you how to fight bosses, and auction house scanners, all of which can make the game more enjoyable but also take away any effort on the part of the players and in my opinion it changes the style of the game to one that requires little effort on the players. I hope these apps do not make it in game.
I'm going to play devils advocate here for a minute. (except for auction house scanners and economy addons, a single player should not be able to control a servers economy with 30min of work each day may they burn in ****)
Do addons that tell you how to fight bosses, allow you to heal by clicking and spell hint addons make the game easier? Or do they allow developers to create more engaging bosses?
In my personal experience, which spans most most MMORPGs that have seen wide release in NA, ones that allow these sorts of addons have more engaging and difficult endgame content. Let me take a step back and define what I mean by that. I consider engaging combat to be an 'ability' rotation you use to optimize either healing/dps/damage reduction, which changes based on circumstances and has decisions a player needs to make to play optimally. You don't need lots of abilities for this to be true, but you shouldn't be able to go into any situation and hit the same buttons in the same order and be playing optimally. I consider this engaging because it allows for you to feel accomplished when you managed to maintain close to optimal performance while engaging in the next key point, difficult endgame content. There are lots of ways to measure difficulty and I'm not sure it is something that can actually be done, but I know what I personally have enjoyed in the past. A good 'boss fight' is one that you aren't guaranteed to win just by showing up with the right group assortment and requires all of the members to rely on each other to perform optimally in a situation that constantly strives to prevent them from doing this through boss mechanics.
I don't actually think it is addons that make any of these things possible. There is no reason a developer can't create a good healing interface or good boss cues. However, what addons do allow for, is for the community to extend the interface of the game in ways the original developers didn't even think of, and while that can result is what many people consider an ugly interface, at least that interface is what that individual player prefers.
Should XIV allow addons?
Unless they can find some way to allow PS3 users to use them as well, they need to avoid allowing them as it would make PS3 players second rate citizens of the game. However, if they do go down the path of not allowing addons, they need to be very receptive to player criticism and ideas for extending the interface to allow the game to grow and mature.