I see. I didn't think we were talking about continuous controls. The behringer fcb1010 I have actually allows you to assign two values to a switch that it will toggle between (i.e. press 1x CC7 goes to 78, press again, it goes back to 24). This allows for something like a volume boost, or a longer reverb when the solo comes up but there are lots of ways to achieve that, so I think it shouldn't be prioritized.
OTOH its not impossible to implement press and hold behavior on a continuous plugin. We do that all the time on our products at work. 1 click would raise or lower the control by 10% of its range, but holding would make it slowly raise up to the max value. OR it could even oscillate up to max and back to min on press and hold. Release the button when it reaches the value you like. TBH though, how useful is that really on a stage? Just use a tablet to change it or a pedalboard preset when that feature arrives. I think it would be a nifty feature that wouldn't ever actually get used.
As mentioned you've argued about it internally, but I think it should be a pretty low priority.
For my plugins no. In many instances it is unlikely the user wants momentary behavior (who wants to hold down the button the whole chorus they want that overdrive?), but it also wouldn't break anything if they set it to momentary. To the plugin it is transparent, it just offers a convenience to the user if its an effect that is used for only a short time or you want an exact press length (like a 1 button looper). I believe there is already the designation pprops:expensive that indicates if its a bad idea to change the value too often.
There actually already is a property pprops:trigger which could make it a momentary switch, but then would the user be able to switch to "latch mode" (toggle) if they want? (I actually haven't added this property yet to my plugs, but I will).
I think a good user experience would be if you map the button to a lv2:toggled control it defaults to a toggle switch (or a momentary if its got the trigger property), then you can select in a side menu or context menu or something (I haven't used the modUI much yet so its hard to propose something) to change the switch type.