That's a good point. Oil is pretty great utility; it avoids the blocked status drawback the other elemental sources have.
I'll have to try using more of it in conjunction with summons.
It's certainly a better tradeoff of damage for utility with Summons than the other elements.
However, you have to have a source of Oil to use it for Summons, and so you could just use that source for the debuff and keep your summons on Blood for their increased damage/health. It's not like you can stack the Oil debuff.
As well some enemies have much higher physical defense than magic defense (and Vise Versa). Which is where having a magic assist comes in handy.
Sometimes enemies have a disparity, and this can be used a lot better if your party has high single-target damage sources (Warrior bursts the Mage, and vice versa); but if you're a mixed party using AoE you'll just end up eating through both armors eventually, which is just giving the enemy more effective-health.
It's harder for the whole team to gang-up on one target if they're collectively hitting different armor sources too, especially true with bosses, as you all likely will be fighting the 1 target together. I tend to favor "Piercing" weapon enhancements over elemental because of this.
In these cases where it's not-so-helpful, using a single and higher damage source is preferred. It's not like having all Physical is particularly bad at killing any current enemies.
When I first tried summons I used the elements and I constantly would have summons hitting enemies with no remaining Physical and lots of remaining Magic armor and I couldn't get through. The rest of my party just removed the Physical Armor much easier. I felt like I had a mixed damage party too. Mileage may vary, but that was my experience.
In general dimensional bolt is best when your enemies don't have any significant resistances, and there's no chance it will set off some chain reaction that will hurt your allies.
But I could see dimensional bolt getting a slight damage buff. If something is unreliable, something else needs to compensate for that randomness.
Perhaps the bolt could be smarter about the element it chooses. Lowest enemy resistance would be a nice touch.
