Last time Bungie talked about the IB power curve (around TTK) they explained that a difference of 10 Light equates to 2% in damage. That's not nothing with damage so finely balanced across weapon archetypes, but yeah, if you're 390+ IB is no big deal.
Clearly you're in that position, but consider the rest of the player base. Many aren't anywhere near max Light, and to them IB looks pretty intimidating. Personally, I know I've noticed the difference in IB early in an expansion, or on an alt, before I got my Light level up.
But hey, these things are relative. IB has nothing on Trials, for sure.