#WebFonts extension does one thing well; it provides web fonts when they are available for the language selected in the user preferences.
That is great for an initial roll-out because it provides a service to people for whom a wiki is intended. Amir argues rightfully that there is a need for web fonts when a text is written in a language where we know that fonts are likely to be lacking.
In some Wikipedias there are templates that identify a text as being in a language different from the language standard for that wiki. Such a template can be extended with an optional web font. It will work when the extension makes the web fonts available.
The problem is that there will be a need for one template per language and such a template will be needed in every Wikipedia. While Amir can do this for a Hebrew wiki, he cannot do it for all wikis; it is just too much work.
It makes more sense to include the ability of identifying a language and selecting a web font in an extension. This could be WebFonts v2.0 and, there is already a request for it.