Guaranteeing a proc is a gain of 10 potency and half a mana. 10. That's it. It's literally just 10 potency.
Getting a swiftcast, alone, without considering mana gains, is a gain of 30 potency, and a full man.
So let's do the math.
Old Acceleration: 3x[0.5, 10]=[1.5, 30] as well as drifting issues if you had procjam going into melee phase.
New Acceleration: [0.5, 10]+[1, 30]=[1.5, 40]+ better utility + charge system makes drifting much less likely.
3 charges of accel existed to do one thing, make your melee guaranteed at a specific time in your opener. Now that the manafication change made melee guaranteed whenever you want, acceleration no longer needed that use case, and thus it needed a new use case to reflect the new RDM's changes, one that happens to help RDM with it's biggest weakness: Pantokrator-like mechanics.
The new acceleration is better in every way.