I think I read in Jacobson's BAII (now available from Dover BTW), "Of course, every irreducible module is completely reducible." A sentence that has a problem if only read in English (versus math). Often, "of course," "obviously," and "it is easy to see that" are used solely for the purpose of not starting a sentence with a symbol, or keeping two symbols from being juxtaposed. And sometimes it is important to say something is obvious or a matter of course.

It is good to use the phrases frugally. And it is good to beware when an author uses them. Of course it is obvious that these turns of phrase are useful.