You certainly have a point: it doesn't make sense for all kinds of protocols. I would say that coins are a good fit for contexts that involve decentralized service provision of some sort. Also in situations that could benefit from a decentralized database (blockchain), though those are surprisingly rare.
There actually already exists a coin connected to messaging protocols, Kin. I don't see much practical use for it however.
This feels like it’s going to provide a horrible amount of friction as a user. It’s basically adding multiple use specific currencies to everyday life. I predominately work across pounds, dollars, euros and Icelandic krona the latter for day to day and the rest for occasional transactions. If I have to add to that multiple service tokens just to access storage or chat or services x, y and z it adds a whole lot of trading and market following overhead. Particularly given the volatility we’ve seen in crypto currencies, especially amongst those with niche uses. Further following Bitcoin it seems like the decentralisation aspects are theoretical rather than practical as those able to provide a service such as mining seem to do better at scale and end up centralising anyway.
There actually already exists a coin connected to messaging protocols, Kin. I don't see much practical use for it however.