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The main reason for use open protocols is freedom. They don't tie you to a specific program that only is available for one (or two platforms) nor forces you to have an account (and accept a contract) with a particular provider.
With a closed platform it can happen that you have to run a bloated client on your old computer. Maybe you don't like the look and feel of the program, maybe it has annoying ads that eats lots of screen space, maybe you run a minoritary platform and the provider haven't released a client for that.
In that situations you usually have other choice than using the official application. Maybe you are lucky and there are third party applications developed using reverse engineering taht meet your needs. But these application most times have less features than the original client and may don't work properly because the developers haven't access to the information about the inner working of the protocol. Even more, the provider could change the protocol breaking non official applications.
With a open protocol all its different. Developers can made applications without have to get an autorization of a company, and having full access to the protocol documentation. They have easy access to understand the inner working of the system and they can develop better applications. Users aren't tied to any program, if one doesn't fit their needs, they just change to another one. All developers have the same opportunities to improve their applications.
