I would just like to chime-in. Note that I, too, am a sysadmin by right and have not much experience in Linux (I can understand some, but mostly through searches and forums, am I able to overcome some difficulties). That said, I also need to add that these are the very reason I am looking for UTM or GUI multi-in-one server.
While we all agree that the feature requested seems to have a point, there is also another, more distrubing consequence should an inexperienced user tried to access and misuse it. If, for some reason, the user knows that he needs LVM, I think he is of more advanced nature, and thus, not having to be afraid of working with CLI.
Should this come into fruitition, I think there should be a warning at the top of the admin GUI Disk Management page (perhaps with a background of red). Also, a warning before proceeding with the task. Just my 2 cents here. It addresses the feature being accessible via GUI and also addresses the danger of using the module by means of very visible warning.