This is really where I come from on this. Like I posted earlier, I'm at 155/200 with only one or two crafting materials present. However, here are some very logical reasons why the idea is not really as great as people seem to think along with some alternatives that seem like rational options.
1. Inventory - If you take the same approach that I do which is to gear to the best of your ability you will be upwards of 130 slots for all 50's and all crafting jobs. While you could store crafting gear and less common jobs on a retainer the UI and server really make this a pain. One of the attractions of the armory system is that you can change "on the fly" and repair in the field or craft during downtime between events. The game really shouldn't be a "hurry up and wait" process and these nuances really serve to keep people busy and entertained.
2. Increased Output - The key benefit to the gear swapping in XI was that productivity could really be maximized if you a) knew what you were doing and b) had the resources (mainly time) to invest into getting the right gear. This gave the game a really deep layer of depth and a sense of motivation and accomplishment. Swapping COR headpieces to shoot on the go and get the most out of elemental cards was great. So was swapping the 20% increase you could get as a SAM if you made the right swaps. But is this a necessity? Not really. It can be argued that the same mechanics can be put into place with the materia system. For example, someone with 3 or 4 tier IV STR materia on a handpiece is obviously going to really make their damage "pop" and this can be achieved without swapping gear. At the same time, the resource investment and level of "l33t" is going to be present. The average player doesn't have the amount of time and gil to buy/craft a ton of hand pieces and obtain all the required materia needed to make a successful piece. This argument actually supports the idea that exact same mechanics are already present.
3. The Macro System - There are 10 slots per macro. While I loved the depth that gear swapping brought I can tell you that having to hit one macro to swap to my WS set, followed by a macro to buff/WS, followed by another macro to swap back to TP gear was ridiculous. We have 10 slots as opposed to XI but this is still going to be prevalent. Also consider the inconsistency of macro swapping as it is now - I often hit my job swap macros only to have one or two pieces get skipped because the UI is not optimized (maybe 20% of the time.) So does a 1/5 failure rate really seem productive? I don't think so.
4. The Skill Factor - Okay, let's put this one to rest - having better gear does not = having better skill or a firmer grasp of the game - it indicates that a player either got lucky (I went 3/4 on Eastern Shadow) or just had the resources (again, gil and time) to obtain the optimum items. For people that suggest swapping the heavy darklight armor in for voke, flash, etc. we know this requires a speed run win of Vale. And yes, that requires some skill. But, at the same time, a random person could get into a pickup group and get the drop so it is a function of luck and resources. I have known many players in XI that, while not having the essential build of gear, outparsed and outworked the players who did because they knew their job better than others. This is like the SAM with every possible piece - they may nail a WS for 30% more than everyone else but they don't do a lot of good if they pull hate and croak. I have seen tons of players without the best pieces perform better than those who possess them and I have come across more than enough people who were decked out that actually had no understanding of emnity or party dynamics. The idea that having more resources makes someone better is laughable - it only increases the probability and even then it is not a dramatic gap.
The conclusion indicates that the current system (materia) possesses the same qualities that people want from gear swapping. Maybe the request should be for more uniquely dominant gear instead of asking for a bunch of situational pieces - gear which can have materia stacked on it. We don't need a belt that adds emnity +40 on it - we have the sentinel's plate belt which you can +50 to. If anything, let's expand on an already great system that will appease both sides.
If you have a counter-point or something that was not touched upon please bring it up. But so far I have really only seen a small handful of valid arguments for and against the idea.