Thinking caps aren't all that terribly rare in Ironman, but you have to wonder if they are worth identifying. After all, they only have durability one, so if you take even one hit (melee, arrow, or spell) that stuns you you'll stand a chance of losing it forever.
And not only that, but the Thinking Cap adds +2 to spell levels of spells you cast. But if you don't KNOW the spell in the first place, the Thinking Cap doesn't help you LEARN it in order to cast it.
So, doubly useless, right?
Well, that's not the entire story. Let's take a look at that +2 to spell levels aspect first. It doesn't say that it will add 2 to spells already known, it says it will add +2 to spell levels for anything you can CAST. There are several ways to cast a spell, of which knowing the spell by reading a book and casting it from your mana-store is just one method. There's also casting from scrolls (for which you have the required Magic prerequisite), casting from staves (for which you have the required Magic prerequisite) and casting from Unique staves. Now why separate the second and third issues? Because when you find a unique staff, a.) the spell woven into the staff will automatically display when you ready the staff, and b.) there is NO magic prerequisite you need to fulfill in order to ready a Unique staff (perhaps a Strength prerequisite, but no Magic prerequisite). Then means that if a Rod of Onan drops in your IM game, even the lowly Warrior will be able to ready it and cast golem from it.
So, this means the Thinking Cap could potentially help increase the efficiency of any spell --- offensive or defensive -- that rely in some way upon slevel. Next question: how many spells rely on slevel in some fashion?
Well, it's easier to ask, "How many spells DON'T rely on slevel in some form or fashion" because it's just that prevalent. In Classic Diablo, the following spells are NOT affected by slevel: Apocalypse, Identify, and Resurrect. Every other spell is affected by slevel in some way; either in damage done, duration of spell, number of spell effects, speed of spell, or mana cost of spell. So, in general, it's better to cast the spell with an identified TC on rather than with it off, with the only exceptions being Apocalypse, Identify, and Resurrect.
But is that really how it works? A small test on the workings of the Thinking Cap to test its abilities. I took a level 20 warrior with no known spells but wearing +magic jewelry and had that warrior cast a Fire Wall from a scroll. According to Jarulf, the spell should last [8 seconds + 8 (slevel) seconds] = 8 + (8x0) = 8 + 0 = 8 seconds, and sure enough, each cast lasted around 8 seconds. Now add a TC and we'd have duration 8 + 8 (0 + 2 [due to TC]) = 8 + (8x2) = 8 + 16 = 24 seconds, and again, that is the noted effect when tested. Thus readying a TC let's you cast a slevel 2 Firewall from a scroll, even if you DON'T know the FW spell.
Then I went to Adria and bought a book of Firewall and read it. The slevel 1 Firewall cast for duration 16 seconds, while readying the TC to drive slevel up to three increased spell duration to 32 seconds. These test results were the same whether the spell was cast from a scroll, staff, or from memory and mana-store.
Now for the more important test. A Mana Shield cast from a scroll when the spell is not known is cast at slevel 0 and stops no damage. However, a Mana Shield cast from a scroll when the spell is not know AND A TC IS READY casts at slevel 2, and stops 1/3 damage!
BUT, and here is the important stuff, test results revealed that if you cast the MS from a scroll without knowing the spell but having a TC readied AND THEN REMOVE THE TC AFTERWARDS, then the Mana Shield still functions as a level 2 Mana Shield, stopping 1/3 damage, without risking the TC being lost to wear and tear!
When is this MOST important? Can you say, final combat versus Diablo? There's a big difference in the damage Diablo will do against you if you have a greater than slevel zero Mana Shield ready or not! Plus, if you saved some MS scrolls (a number of which drop in practically every IM game), every member of the party can go up against Diablo wearing an slevel 2 Mana Shield, thus stopping 1/3 of the Demon Lord's damage every attack! (Mana Shield scrolls have no magic requirement to cast.)
How would you like to go into the final combat with Diablo only doing 2/3 of his usual damage (20-40 versus 30-60)? A few of his attacks won't even stun the higher clevel characters!
Not bad for a durability 1 item.
Now for general area where having a TC on is going to help.
Area #1.) Increasing spell damage: Bloodstar, Elemental, Fireball, Firebolt, Flash, Guardian, and Nova.
Area #3.) Increasing the number of effects created by the spell: Chain Lightning, Charged Bolt, Flame Wave
Area #4.) Increasing the speed of the spell, (making it less likely to be dodged): Fireball, Firebolt, Holy Bolt
Area #5.) Increases the power of a healing or defensive spell: Heal, Heal Other, and Mana Shield.
Area #6.) Increases the duration of a defensive spell: Infravision, Stone Curse.
Area #7.) Decreasing the mana cost to cast the spell: ALL spells except Apocalypse and Nova.
And finally, some specific situations where spell-casting while using the TC will come in very handy:
Situation #1.) Mana Shield
As per above
Situation #2.) Heal and Heal Other
Not only will readying the TC make casting less costly to you if you happen to know the spell, increasing the slevel of each spell INCREASES the amount of healing done. So again, wear that TC if you are planning on casting a Heal or Heal Other, OR when you are planning to Heal yourself using a Heal scroll.
Situation #3.) Firewall
Increases the duration of the Firewall spell by 16 seconds; great for the Butcher and Leo and any boss (like dogs) or foes (like witches) that sit still when attacking.
Situation #4.) Golem in endgame
Usually two to four golem scrolls drop in a game. Have one ready for Diablo and the endgame, and be sure to have the TC on when you cast it.
Situation #5.) Holy Bolt in endgame
Once Diablo gets engaged in melee combat, he'll start getting stunned and thus won't be able to attack. Slap on that TC and start spamming out the Holy Bolts! You risk losing it, but hey -- it's endgame! Go for it!
Situation #6.) Mage conserving mana
Any time a mage casts a spell and wishes to conserve mana (such at Town Portal, TK, etc.), the spell will cost less mana to cast with the TC on rather than with the TC off.
So if you can keep that TC tucked away, there's plenty of situations where it will come in handy in IM, even with duration 1/1. You just have to be very very careful.
PS Yes, I realize this forum is dead/has been moved. Since Classic Diablo is almost dead too, pretty much the perfect place to post this.