We experience concierge (now club level) for the first time on our last trip in March 2006. We loved it and now my husband refuses to not go concierge on future trips.
For us, it was worth it, because at the time we booked it, I had just recently been implanted with a Cochlear Implant and still hadn't gotten to the point where I could talk to strangers on the phone again. Being able to plan my ADR's, etc. via email with concierge was a total godsend for me. It was nice to be able to decide what we wanted to do and put in our requests for it 200+ days out, knowing they would be fulfilled. (and they were)
For my severely ADHD son, the Poly concierge lounge provided a sense of consistency & routine, which then gave him a feeling of security. He was able to go up to the lounge to get the same breakfast every morning and to make hot chocolate from the machine (which he absolutely LOVED). The evening hor d'oevres were the same for the children every evening, which of course were important to him (ADHD kids thrive on routine!). He ate very well, which was a blessing to us as his parents, because it prevented meltdowns. (he had only one... when he discovered that Disney didn't have Cartoon Network! LOL)
Also, the location of our concierge level room was a plus and worth the extra money, because our children played together on the beach and watched the fireworks there while dh & I sat on the patio and enjoyed each other's company with a drink from the lounge.
My husband still gets tears in his eyes when he recalls our son telling him that he loves watching the fireworks (and parade) on the beach because "no one bothers me there."
So yes, for US, concierge is well worth it. Because of the expenses, we can only go about every 2-3 years. But that's okay with us.
If you're asking us to justify the expense of concierge, I can't help you there. For us, life is too short to worry about how much is spent on a vacation.
It's all in perspective. For some, it's not worth it. But to us, it is.