I tried this with Daisy Walker and +5 is useful. There are many things you can't afford to test twice like sticky curse treachery that needs an just for a chance to have a go.

Or plainly for Ward of Protection-like experience on fail-by tests that can be used to help others while grouped up. Baked in the game rule it limits helping commit to 1 card, meaning that it is an ideal use case of a card that gives a lot of icons in one.

In a pinch that you really need Deduction to activate on high shroud location the +3 side can also be used. In turn this can save deck space since it is like 2 utilities in one. It is quite a swiss knife for her.

Speaking generally for all other cards in this series, there is no need to worry about incompatibility with other skill shenanigans. This series of cards are "power helping commit" (not for yourself) that gives a ton of icons and can flex to 2 icons all in one. You should play like it is designed, that is to help others so only your single commit is enough to pass. It can also restore turn flexibility in spending Activate action to have a go at critical scenario tests when racing against doom. e.g. I want to use 3 actions to setup, but I am also the one with high to pass the test at the current location. I can let others test instead and pitch a single skill card, appearing as if I am the one testing, as the card "draws power" of investigator flexing the skill.

In my opinion to justify including one :

• Willing to stick to others more than usual, at the same time having other same-location team play cards in the deck. (My Daisy Walker will be using Old Book of Lore and Encyclopedia to prop up the other investigators and also having Disc of Itzamna to catch spawns, so Occult Theory feels great to have on hand. Or maybe Guardians that aims to use Taunt, Stand Together, "Get behind me!", Teamwork, etc.)
• Running out of deck space to include components for your main combo, needed to compress skills to a few cards.
• or that the card is covering must be able to get +4 at least to worth it, since these 2 stats are abundance in treachery test where likely others can spontaneously benefit from a single helping commit each mythos phase. The stat pairs chosen for these cards aren't random, all cards in this series touch or one way or another (but not both), highlighting the treachery-passing power of each one. Meaning that you should find someone with 4 or 4 to translate to those stats and can include the right card.
• The other side not or should be able to get +3 at least, otherwise it is not flexible enough to worth the XP.

It is not easy to find someone that can actually get +4 on or while having +3 on the other one. I just look through all investigators and filtered all the +4 / users with at least +3 on the other side (in parentheses), for your reference.

• Defensive Stance : ()
  • Mark Harrigan +5 (+3)
  • Lily Chen +4 (+3)
  • William Yorick +4 (+3)
• Survey the Area : ()
  • Monterey Jack +4 (+5)
  • Trish Scarborough +4 (+4)
  • Ursula Downs +4 (+4)
  • Rex Murphy +4 (+3)
• Occult Theory : ()
  • Norman Withers +5 (+4)
  • Daisy Walker +5 (+3)
  • Mandy Thompson +5 (+3)
  • Gloria Goldberg +4 (+5)
  • Marie Lambeau +4 (+4)
  • Carolyn Fern +4 (+3)
• Dauntless Spirit : ()
  • Daniela Reyes +5 (+4)
  • William Yorick +4 (+3)
  • Tommy Muldoon +4 (+3)

Despite being a Practiced skill card that can potentially give upwards of +4 to +6 to an investigator's Will or Lore, Occult Theory is largely made irrelevant by its restriction on only having icons when it is still in your hand or committed to a skill test. Since Occult Theory has no icons outside of those select circumstances, Occult Theory becomes a dead card when attached to cards like Astronomical Atlas and is never a valid target for events like Practice Makes Perfect or even Written in the Stars (since those cards have no icons while they're still in your deck). The card is still potentially playable as a Hail Mary for Investigators that need flexible skill dedications, but there are no actual combos or tech involving Occult Theory when Mystics already have access to numerous great Level 0 skill cards.

At the same time, Occult Theory compares poorly against Level 0 Mystic cards and doesn't provide good enough stats to meaningfully compare with other upgrades that cost experience. Investigators with access to Mystic Level 0-1 who need a large dedication to pass a Will-based skill check can just cancel treacheries like Ward of Protection and Deny Existence, while Mystics who need to gather clues can just use their will to investigate using cards like Rite of Seeking or Read the Signs rather than try to boost their Lore to compete with Seekers. And even in the event that an investigator still desperately needed a flexible skill dedication, Mystics already have amazing skill cards like Promise of Power that give +4 to every stat that they can filter out of their deck with Practice Makes Perfect or Astronomical Atlas. Occult Theory is the answer to a non-existent problem for Mystics, and even in the event that an Investigator wanted to try the card out, they should already have access to strictly better option within the Mystic card pool.