The answer is very simple.

Think of your constant spell casting is hits you take to your HP and you never once popped a HP regen CD that has a 2 minute CD, which is the only thing you have for recovering HP. If you pop a mild HP regen as low as below 33% long before the fight is over, then you are going to die. Maybe not faster, but you will surely go OUT OF HEALTH. This could have been avoided if you had used your HP regens sooner and more often.

Now replace all the HP with MP.

Notice what I did there? It's the same thing, and it's frustrating to see other healers fail to understand MP management. Especially ASTs. AST is a complete noobtrap.

The goal is to use the MP refresh as many times as you can. The goal is to keep your resources up, like everyone else. If your in a boss that usually lasts for 6 minutes then you should have used it at least twice. It's not meant for emergencies like the Bard's MP Song. You can't be neglecting it only to use it below 33% while spamming heals every second, because the MP refresh, just like the WHM's, is weak. The constant drain will often if not always beat the Refresh, which guarantees bad effectiveness when you use it at a low threshold.

Does that mean use at 100%? NO! Going to the wrong extremes are horribly bad, and you should know better!

As an AST I never have MP issues as many ASTs love to claim and yet everytime I'm not the healer, the same thing happens everytime I'm with an AST they have issues thanks to their neglect. Some of them are even worse where some store an Ewer (MP Refresh) card only to never use it on even themselves.

I use my MP Refresh whenever I get to 66% long before the fight is over, and by the end of the fight I'm ready for another pull immediately. While many if not most of the ASTs I had the misfortune of being subjected to need breaks, after just one normal group. I'll be lucky if I see an AST that doesn't act like the very concept of MP management is new, much less have ANY faith their skill to begin with.

Outside help, such as bards are completely irrelevant for this. The game is built so that we can take care of ourselves as ASTs, because of the possibility bards are not in the party.

Why is it a popular thing to use MP Refresh at horribly low thresholds, only to fail? What loser guide did they read!? WHERE'S THE INFLUENCE!?!? WHY NO EWER!?!?!?!? WHY IS IT SO EASY TO FACEROLL AND STILL DO BETTER THAN THE BAD ONES!?!?!?!??!?