I think there are a variety of contexts in which more than one form of address is acceptable. For instance, I think I have always been addressed as Mr + Last Name when in an airport context. I think it fits. In a phone support context, I don't feel comfortable hearing a CSR refer to me in that formal manner repeatedly, and I would also prefer not to hear sir as a tag to every other sentence (one will do nicely, ha ha). That's putting myself in a customer position.
When I put myself in a customer service position, I think I have just been in situations where using a first name was the norm. I have used Mr/Ms on a few rare occasions in spoken language and I recall using Mr with someone who was the owner of a company. Not sure why I did, maybe it was because I thought he had encountered a delay or had gotten a runaround before landing with me, and I guess the Mr came out instinctively to show him I thought he was important.
So I am seeing both sides here...I use first name the majority of times in speech and writing. I wouldn't say that the first name usage means they are run of the mill. When I think of a successful handling of a customer, it is about speaking in their language, understanding their issue, and being the answer to a need, like a hand fitting in a glove. And you can achieve that in a conversation/customer service situation without using Mr/Ms, by demonstrating acknowledgement, being armed with information that they need, etc. This is what I have experienced in a North American setting.