Let's say we have a 15% natural crit chance (enough crit for 50% bonus damage) and a 15% direct hit chance. You have therefore a 12.75% chance Crit, a 12.75% chance to Direct Hit, and a 4.5% chance to Direct-Crit. This grants a total bonus of 11.8125% damage, up from the 11.25% damage they'd do if they could neither stack nor conflict (until each over a sum of 100% chance). Thus, the multiplicity is responsible for exactly 5% of their damage bonus.
Now, let's say we increase the Crit chance by 20% via Devilment. On its own, at our current Crit Bonus, this should amount to an increase of roughly 10% damage if there were no multiplicity, for a total of some 21.8125% damage bonus. Instead, it gives 22.5625%, 0.75% higher.
Now let's say we have the choice of an added 5% damage or an added 20% Direct Hit chance (raising each to 35%, which is the best case scenario for multiplicity).
Damage: 28.690625%
Direct Hit: 29.3125% (~.62% higher or a relative increase of 2.2%).
But let's pretend for a moment that 35% of each chance is just such a pitifully small stat value that we just can't yet see the miracle of multiplicity reach its full potential, to where it could really outshine there mere +/- 5% damage modifier attached to every skill. In fact, let's go to the maximum: 80% of each chance naturally, or 100% each under Devilment/Spire.
Starting from the Devilment alone: 72% damage increase.
Adding Spire: 100% damage increase.
Adding Damage: 80.6% damage increase.
Finally, we can see multiplicity pay off. But, if that's what you're hoping for from XIV, you really ought to consult our stat charts: you would need over 4500 Crit just to reach a 30% Crit chance. You are unlikely to hit any point at which even the optimal addition of Direct Hit chance (raising it to one's already buff-inflated Crit chance) would outperform its nominally equivalent basic damage buff by more than a few scant percent. And, as they tend only to follow multiples of 5 once dealing with relatively large numbers, unless Direct Hit is balanced for nominal equivalence (that is to say, without relying on multiplicity whatsoever, a (Direct Hit) Spire will never outpace a (Damage) Balance within the lifetime of this game. A 15% added chance of 25% more damage and its Crit-multiplicity will never compete with a 10% Damage buff within the gearing lifespan of this expansion, even when stacked atop CS, BL, and BV, simply due to how limited our stats are.
Direct Hit is therefore, for all intents and purposes, just another Damage bonus, effectively indistinct from Balance.