What level of proficiency have you achieved? As a B2 in English, I cannot fully agree with you.

The method you pick depends on your goals of learning the language. If you want to simply read and listen - sure, your method might do.

If you want to actually communicate in a language with native speakers, especially in a business environment where every mistake can cost you good impression, then you'll need some information which can be really hard to take from something else than textbooks. For example, conjugation tables, which can be quite lengthy for languages such as Russian or Arabic. Textbook creators have put some effort to make these tables, and I don't see any benefit to discard this effort.