Tried a target dummy test with and without fel eruption to see if there is a DPS difference.
Without Fel Eruption: 5 minute attack
Damage – 14,903
With Fel Eruption: 5 minute attack
Damage – 16,012
Used an agility potion which was active for both.
I was well-fed.
Despite sharing a cool-down, the benefit to fel eruption which gives you more DPS in a raid encounter is being able to apply an additional mechanic to ranged adds which you would otherwise be forced to swap to, therefore dropping damage on a main target and inevitably dropping DPS.