> Social conventions aren't established by protocol drafts, they're organic.
Organic stuff lends itself better to optimization - by constant feedback.
I often have to write FULL responses like the example you provided and find this incredibly annoying. There are many reasons to that. Sometimes, I just don't have time to craft a nice sounding and formatted answer at the moment (if I'm going FULL, I won't tolerate any spelling mistakes, typos, double spaces, etc. If one commits oneself to quality, it should be 100%).
But sometimes, the crafting of the answer FULL requires too much cognitive power. Quite often in situations similar to your example, my real reaction is a linear combination of things like "I'm not sure if I have time", "I wanted to go to a party on friday", "I'm worried what my girlfriend will think of this", "I really need some rest", "I'm worried we won't have much to talk about and it will feel awkward", "I'd really like to save this money for 3D printer electronics", "goddammit, I'll break my ketosis again if I'll eat a normal lunch with you", etc. It takes a bit of effort to craft a message that is a) nice, b) not an outright lie. A bit of effort I don't always want to make at the very moment.
So yes, I'm all-in for any kind of social protocol that would allow me to decline/postpone requests without having to explain myself to other party just for the message to sound polite.