It's not that I don't see any advantages at all: there is definitely something to be said for making it easier to separate looks from stats. I remember when the Thermal Retention set first came out - I thought that my character would look great wearing that while riding her tauntaun around Hoth or Ilum! But would I want to wear it the rest of the time (especially on Tatooine...)? No. So I wasn't going to move all my mods into it, and wearing a full set of gear with no stats was pretty much out of the question as well, at least if I wanted to do anything but strike a quick pose for a screenshot.
With the outfit designer, silly ideas like this - costumes that you may only want to wear temporarily - will be able to thrive, and my inner roleplayer definitely appreciates it.
There will also finally be a purpose to all those green, blue and purple non-moddable pieces of gear again that you find out in the world or get from crafting. They had become pretty worthless except to gear levelling characters or companions about whose looks you weren't too concerned.
But at the same time I can't fight the feeling of how utterly redundant this whole thing is. The whole point of moddable gear has always been that you'd be able to keep a certain look while upgrading your stats. This is why my Jedi and Sith characters still own the lightsabers they received early on during their class story. If we now have another system that allows you to separate your stats from your looks, is there really a point in having mods anymore (aside from the fact that the outfit designer currently doesn't work for weapons and companions)? I mean, I suppose that mods will continue to exist as long as Bioware continues to release high-level gear in the form of moddable pieces, but if Bioware decided to make all gear unmoddable with 3.3, I wouldn't think that anybody would mind very much except some min-maxers who couldn't pick and choose every single part of their gear anymore. In the meantime we'll be left with a system that will make gear management even more cluttered than it already is. I still remember being confused by the concept of mods when I was a new player myself; I can hardly imagine how confusing this new mish-mash of systems is going to be to newbies. This is not good game design.
Also, while I'm generally very annoyed by people who criticise parts of the game simply for not being what they hoped they were going to be, I'll allow myself a bit of hypocrisy in this case and express annoyance that a feature called "outfit designer" will not actually allow you to switch between different (statted) outfits. Now there's something that would have been genuinely useful! As someone who does both PvE and PvP, switching between my two gear sets is always a nuisance (especially with the implants and relics, since you have to drag them into exactly the right place), and the amount of blunders I've made while swapping gear sets is much too high to count by now. Making that easier would have been something I would have appreciated with no reservations.