Simple rules (which, being me, will turn into a giant post anyway):
- If a mage can safely give you a sneak/invis to get you going at the zone line (which they usually can: rare is the zone line where it's unsafe to cast), by all means, they might as well. There's no need to waste anyone's money if the only concern is saving MP.
- If *all* of the following statements are true in regards to sneak/invis in a dangerous spot, then the mage should, again, take care of it, because the only thing that's lost is MP. If not, take care of it yourself with items.
1. The mage is not putting themselves at risk for your benefit. If there's a magic-aggro nearby (which *will* aggro the casting) or a sight-aggro nearby (which *may* aggro the mage when their invis drops upon casting), the mage should not risk (or guarantee) death to save you gil.
2. You are exercising common sense. Don't wait for your sneak in the middle of a pile of Robber Crabs, for example. And if you expect a mage to babysit you with sneak/invis, it's pretty damn stupid to run way off ahead, rather than sticking close to the mage.
3. You understand how sneak/invis *works*. It gives warning messages before it conks out. Take this time to find a safe spot to recast (or be recasted on), and if you're having a mage cast on you, let them know *at the first warning*. The mage does not get the warning messages except for *their own* sneak/invis. Demonstrate that your brain has cognitive and communicative functions, and *tell them*.
Also, the old sneak/invis effect needs to be gone before a new one can be put in place. Whether you're putting the effect on yourself or on someone else, know how to cancel your effects. There's also the recast on Sneak/Invis to consider. If a mage has to Invis you, they then have to stand around at risk of aggro for about 10 seconds before they can Invis themselves back to safety.
Side note, which far too people are unaware of:
Since casting invis, using tonko, using a prism powder ... all of these actions will *themselves* cancel invis "by doing something", you don't need to manually cancel. The act of recasting *on yourself* will cancel invis just before the new one lands.
Because of this, there is no excuse whatsoever for getting sight-aggro, unless you have to drop invis for another reason - most commonly replacing *sneak*, when you have to dodge both sound and sight aggros. You can recast invis on yourself right in front of a sight-aggro enemy, and it will never ever see you. (So long as the invis drops due to the casting, and not due to wearing off with time.)
This does not apply to having someone else invis you - because although an invis drops, namely the other person's, it's not the same one as is being replaced, namely yours. And it doesn't apply to sneak at all, because none of these actions automatically cancel sneak.
Other notes/advice:
Sneak and Invis will aggro magic-aggros such as elementals or arcana. Tonko will not. Items will not. In places such as Ro'Maeve or Ru'Avitau, even mages need to use items, even for themselves. And thus they certainly can't use spells on other people.
*Everyone*, mage or not, should carry a full stack of oils and a full stack of powders at all times. (I can only think of one situation where you might want more than one stack on you, and that's going through Ifrit's Cauldron to Yuhtunga for ZM5. And then it's only because you'd be sitting around waiting for flame spouts.)
Keep another full/partial stack in your mog safe. When you get back to town after using a few from your inventory-stack, refill your inventory-stack to full with the ones in your mog safe. When your mog safe runs out, buy a new stack and put it in there.
On any given outing, you will likely use anywhere from 0 to - in a *very* extreme situation - maybe half a dozen of these. Usually 0, sometimes 1-2 on average however. Rarely is it more. It takes a long time to go through a stack, so even though the stack *is* over 10K, it's not like you'll be burning through a stack of items every time you go somewhere.
Even if you do, just suck it up and figure it into mandatory expenses like food and equipment, don't treat it as something optional which you can't be bothered to buy (or bring).
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And Cythe ... if-clause number 1 above applies even if the mage would be putting themselves at risk for the benefit of a tank who can protect them. Because who's to say that mage + tank can duo whatever enemy (or perhaps plural enemies) happen to aggro them? It's very possible that all you'll accomplish is get *both of you* killed, if you tried a stunt like that. (Depends on the enemy that aggros and how strong it is, of course.)
The most you can reliably say is "mage + tank can survive for a minute or two until the rest of the party gets there", and you can't necessarily assume that the rest of the party is a. close enough, b. able to find you, or c. able to get there without aggro *themselves*. So you can hardly depend on that eventuality either.