Paul, your answer is about quickly moving through the list of plugins, not the list of presets of a given plugin, which is what the OP was asking about. For something like a modular synthesizer VST, there might be 50+ knobs/switches to control the plugin, and these presets store the positions of those knobs/switches so you have a reference point to make changes from.
The fact that there is no keyboard shortcut for switching between presets is really a problem for me as well. When you click on the preset menu, it automatically centers you in the middle of the list of presets, meaning you’ve totally lost your place in the list of presets, making you have to scroll down possibly along time to get back to the preset immediately above the one you were just on.
Of course, this is not really an issue for people who are using Ardour just for post-production. But I like to mess around, moving from preset to preset quickly until I find a sound approximating what I want, and I want to be able to do this in real time to not break my “flow”. It would be even better if this preset-switching could be one-click automated as one can do for individual parameters. As it is, it’s easier to switch between plugins than it is to switch between presets in a plugin, which means it’s not really suitable for my use case.
I don’t view this as an esoteric feature. It’s in Reaper and many other DAWs, just not Ardour.