I had seen the Met mezzo Stephanie Blythe in Wagner's Ring Cycle as Fricka the previous year before she played Nellie in the Philharmonic's concert version a few years ago, so I was familiar with her voice. Yet even that did not prepare me for the bomb she detonated the night I saw it. Being cheap, I had bought a 3rd Tier side box high above the the stage. The sight lines were so bad I heard they refunded some money to those who complained. The microphone and sound engineering were terrible and my partner who had never seen Carousel lost interest a quarter of the way through because he could not clearly hear nor understand any of the lyrics. That's how bad the sound was! And then, You'll Never Walk Alone arrived... Blythe's voice has the heft of an 18-wheeler but the supple smoothness of heavy cream. No echo-y microphone could harness her. Her voice rose over the poor mic-ing and resonated through the hall, growing and growing with each verse. By the end, I actually reached out to hold onto the hand rail of the box seat to steady myself because I swore I felt the box vibrating and for a moment actually thought it was going to collapse!
I think i agree with the OP that this shouldn't be a trend where an opera star is hired in a knee-jerk reaction. However, if the voice is suited and the acting chops are there...it can make for an exciting performance. Paulo Szot in South Pacific was genuinely dreamy in the non singing parts and is a great example of perfect casting.