I empathise with these points about protecting people from corrupting their projects with wacky plugins.
However, I've seen plugins (iirc Skymens experimental text plugin, experimental 9patch, even dynamic layers before this was officially added).
If one used these addons, and then a major change occurs with text/9patch/layers, wouldn't we be in the same position as if someone modified an official plugin? Raging bug reports but it all stems from a third party addon.
At the end of the day, Scirra don't offer support with 3rd party addons or modifications (and sometimes even do offer support in these cases but only if common/quick to solve). Whether it's a 3rd party addon or a "experimental fix" addon, or even if it were to be an official addon that's added a 2nd time as a modified addon, it would all fall under 3rd party addons, which Scirra won't support, which seems very understandable.
Whilst I do sincerely agree with protecting people, I guess I feel there's a point where people must be warned about what they're about to do (i.e. Labelling these plugins as experimental), but beyond that they're on their own.
Official plugins could be marked as protected, as to not allow modification (if someone did tweak platformer behaviour, then they'd need to install it as a 3rd party addon then "move their events over" to use this modified behaviour, rather than updating the official plugin - this would be time consuming for some, but prevents "hacky" workarounds and keeps things consistent.
I for one use events mostly, so this doesn't deeply affect me, but sometimes wondered if the community could enhance plugins such as the physics addon.
On another note, I think it's really interesting with Physics - I guess if there was much bigger demand, we could get a physics 2 addon or something? I have hit issues with current physics for many different reasons that could all be fixed by having better collision filtering, and always wondered "well it's within Box2D, I wonder why this can't be exposed in event sheets?", but now this makes sense.