Sounds like reasonable advice, I suppose.About 10 years ago, when my Cayman battery died, I had heard about AGM batteries and asked about changing from wet cell to AGM and the Dealer said he was going to advise against it because people knew when a wet cell battery was dying. You've lived all your life with it. You just know. But AGMs just "died" unexpectedly so he advised not doing it.
In the powersports world, AGMs have been used for quite some time primarily, as I understand it, because they are more resistant to failure caused by vibration, and because they can be mounted sideways, upside down, etc. (a trait not needed in most cars).
Me, as I said before, I'll probably just go (or be towed) to the dealership when the time comes. I can understand people's not wanted to spend unnecessarily, but it's not a big deal to me. What gets my goat much more than the cost of battery registration at a dealership is what the mothership charges for different colors on the dash dials, or leather-covered vents, and so forth, and (apparently) folks still pick some of those options.