My band does pretty much exactly the same set-up as Darren's band does on several of our songs. (iPod with click to drummer and some additional parts including keys and BGV's)
More so our producer's call as he does a lot of elaborate arrangements of key, string, and vocal parts (which are great) and his philosophy is that the audience wants to hear it how it is on the CD or he wants to hear it as he envisioned it which can't always be achieved with a four-piece band.
My feelings on it? Mixed. I like being able to present our sometimes grandiose stuff live while still playing and singing our asses off. (and yes, I truly believe it takes being incredibly "in the pocket" to play well with a click). There's really no other way it could be done on our budget and the stages we play on. We all sing and sing well, but the extra stuff #1: helps "fill it out" more and #2 helps it sound more like our CD. If we played or sang poorly, there's no way those tracks would "help" us.
However, I love what 100% live brings. The spontaneity. The unpredictability. The raw power. I've always loved that Queen, despite their notoriously layered studio work, would play 100% live, just the four of them (although with a touring keyboardist on later tours.) Yes, the arrangements often sounded different from the record, but I personally liked that.
I love that the classic live records often breathed and varied in tempo and arrangement from the original recordings.
I can remember seeing Lenny Kravitz live years ago. I saw his guitarist's (Craig Ross's) mouth moving, but it was totally Lenny's voice/timbre I was hearing and Lenny was nowhere near a mic. When Lenny did sing, I'd hear Lenny singing BGV too, and yes, I felt somewhat cheated.
I've heard through the grapevine of one case of a nationally known current band where the guitarist looks like he's singing backup, but his mic isn't even on!
On a different but related subject, how about the use of AutoTune LIVE these days? I happen to really like The Killers, but bought a live album of theirs and it's pretty obvious they used it on Brandon's voice. Whenever I'd see Gwen Stefani perform live on TV before, she always had poor intonation. Last performance I saw of her, she was spot on. Every note. And I know it's not cuz she's been practicing!