I see what you mean but part of your argument stands and falls with how you construct your example. I would construct it somewhat differently (without claiming that yours is invalid): I would say close to 100 percent of the users understand English, at least to the extent that they can understand the site navigation and other non-content text. For this reason, my site language would be English. But when it comes to the translation of posts, an unknown percentage of users would appreciate having those translated into their native language (using the translator plugin), but for this to be available, they need to change their locale. So this is my main motivation for allowing multiple (ideally: all available) locales.
To explain this motivation a bit more: it is not easy to cultivate a truly multi-lingual forum, especially in this kind of context where English kinda seems to work and thus becomes the norm. I would argue that a major benefit of the translator plugin is that the experience of being able to translate posts from other languages into your own increases the acceptance of posts in a non-English language and hence also users’ willingness to write in their own language whenever they feel uncomfortable with English.
Having said all that, I guess we’re also coming from two different directions (or trying to solve two different problems). You’re trying to find the solution within the status quo so that most users can understand what they see. I’m trying to find a way to ensure that all users see the same information.
My problem is not primarily that of translation but rather a consequence of translation being done, but incorrectly, if I may say so.
Putting it differently still, I’m trying to stay in control of what my website says and currently there is a trade off between being in control and being heard. If I turn off locales, I’m in full control of what my site says, but some users may not understand it. If I turn on locales (even if only a few), I am no longer in control of customized text (unless I translate them into each locale, of course).