They are becoming more popular so there most be applications where the cost premium is justified. In van conversions where the batteries often end up inside, the sealed nature compared to flooded batteries makes a big difference. And if they are placed under the van where access is limited, the maintenance-free nature is then important.
I'm no battery expert, but if I was building a Class B or having a custom one built for me where I could control what was used, I would go with AGM unless I could justify spending even more for lithium batteries.
One issue is that AGM charge best at slightly different voltages than flooded, so if connected to vehicle alternator then charging might not be optimum. Some even claim that it will shorten their life but I'm not sure it's that big an issue.
Also note that Ford Transit now comes with factory dual AGM batteries as an option. I think the trend is in that direction.