Hi Meowy,
In answer to your OP, I'd say it's a time sink, for those that are bored, rich enough, or just gamblers.
I would personally prefer a Long Term, Time Sink Goal that rewards player effort, where everything you do is a positive increment towards that goal:
* Like Final Fantasy XI's Relic Weapons.
At least for Relic Weapons (early days), every time you went into Dynamis, you were making positive progress towards the goal of completing your Relic Weapon (by earning more Special Currency). You were putting in the effort and your work was being rewarded. It didn't have a chance to "blow up" (lose all your progress) only to start over again.
With the Multiple Melding Materia System here, it's far more based on Luck. It's the Player standing around and pulling a "slot machine" or "rolling the dice." You attempt a Multiple Meld, roll the dice, and see if it exploded or passed. Repeat.
I'd love to see Yoshida-san and team allow *some* way to integrate Player Effort and Skill towards Multiple Materia Melding. As I posted before, here's an example:
* Complete a series of engaging, challenging Quests ending with a challenging Boss Fight that allows you to gain a Blessing from one of the Twelve, which can either grant you a Materia Slot or at least gives you a Bonus to Succeed in Melding.
Because at least in this case, the Player is rewarded for making a real effort and utilizing their skill towards this Materia Melding, instead of just rolling the dice in town (or paying someone to try and roll the dice for you).
And most importantly, as you say, Yoshida-san and company are not on a good precedent right now: Our previous "top end" equipment seems to be getting invalidated with every patch so far.
To be fair they were revamping recipes (huge change) and revamping the Job System (which influences weapons / armor), so I'm holding out hope that this is just an aberration and that we *will* get a stable set of Equipment to work towards that lasts more than just ~2 - 3 months.
We'll see where this goes in the coming few patches (e.g., if Moogle Weapon drops or Garuda Weapon drops, or new Recipes in 1.20 or 1.21 just invalidate the best gear we had for 1.19 well...).