I specifically removed mob resistance out of the picture *because* people will come up with marginal scenarios such as "casting ice on an ice elemental on an ice day"--surely a moot argument.
And of course, my thundaga3 comment is merely to poke fun at the all too common mentality of the blms to just cast the biggest spell they've got--no offense to the blms, but you can't deny they're out there.
The case for Ice staff, I certainly agree; harkens back to the days of the distortion craze.
But I happen to know of the phenomenon of the blms going "look at that, i did xxx damage" but being careful not to make comments when they are pitifully resisted. I'm sure you all have seen it too. Now that I'm not at work, I can actually do the calculations:
Here are the base damage of tier 4 spells:
stone iv: 391
water iv: 420
aero iv: 450
fire iv: 482
blizzard iv: 516
thunder iv: 551
(these are base damages calculated at int65 with a 5 difference between mob int (60). if you're interested, you can see the jp calculator here"
http://f4.aaa.livedoor.jp/~rmkta/magic/) The 5 difference, to account for the fact that mahatma will have 5 more int
now, let's see what happens when you add in the day bonus (I did check the effect of the day, and it was 10%) and the af2 bonus (5%), but take away 5 int, simulating the int difference between mahatma and af2:
stone iv: 438
water iv: 471
aero iv: 506
fire iv: 542
blizzard iv: 581
thunder iv: 622
Now let's see what happens when you're using the spells on the wrong days (i.e. days that give you penalties, wearing the mahatma)
stone iv: 351
water iv: 378
aero iv: 405
fire iv: 433
blizzard iv: 464
thunder iv: 495
Let's look at stone iv, the weakest one that no one ever seems to want to use, and thunder iv that everyone loves.
What happens on earth day?
stone iv: 438 (138 MP)
thunder iv: 495 (171 MP)
It still might look like you get a better deal with thunder iv. hey, you do ~57 damage more in the same time span, right? Now, consider resists.
I don't have %s on the resists--but they're a lot higher than the blms would want to admit. Let's for the sake of simplicity, say that stone will be half-resisted 3% and thunder 8% on some hypothetical high-level mob. I would say the gap would be bigger, but it's enough to make the point. Fair enough?
average damage of stone iv: 431.43
average damage of thunder iv: 475.08
Of course, this is only an average case. In the case that you actually *do* get the half-resist, your raison d'etre as a blm is nil.
So let's say you're doing a MB now. Good ol light. You probably aren't going to get resisted on a mb unless it's some hnm, but let's see what we have on the light side. (MB Bonus is a constant factor so it's pointless to include):
case: wind day
aero iv: 506
fire iv: 482
thunder iv: 551
case: fire day
fire iv: 542
aero iv: 450
thunder iv: 551
case: thunder day (moot point)
thunder iv: 622 af2
thunder iv: 551 mahatma
aero iv: 450
fire iv: 482
By sticking to the day of the week, you're doing *at worst* 8.1% less on damage, but with a higher chance that you will not be half-resisted. And, of course, nothing prevents you from taking a chance thunder iv with af2 at 1.2~1.8% less damage as mahatma (as you'll see below)
I don't know--is it as trivial as you guys make it out to be? Depending on the actual resistance difference, mahatma may actually be hurting. And, of course, to finish it off, here is what you get on the off-days with af2:
stone iv: 381
water iv: 410
aero iv: 440
fire iv: 472
blizzard iv: 506
thunder iv: 541
a constant difference of 10dmg--in the case of thunder iv, you lose 1.8% damage. And of course, the higher your actual int is, the smaller that ratio becomes. So, is it worth losing 1.2~1.8% damage on the "87.5%" of the time and getting an additional 5% on the other 12.5% and saving an inventory slot and 2m?
To put this into perspective, the loss of 1.2~1.8% damage is basically saying a 1 hour battle will last 1 hour, 1 min and 10 seconds. I don't know. I say to each his own

To whomever that 1.2%~1.8% is worth the inventory slot and 2m, sure, go for it. But I don't think the ones who choose not to go that way deserves to be belittled.