Native speakers do not learn their native language with the help of an other language. Maybe direct methods are better to reach similar level of proficiency. But books that use a direct method are often hard to understand because of the lack of context.
There are tons of advantages and disadvantages to the direct method.
The direct method is put simply as a teaching method that use only the target language. This itself presents some of disadvantages, such as having the learner to not be able to connect words from their native language to the target language. Slow learners also struggle due to its rather complex learning style so to speak (Wikipedia).
Some advantages are rather interesting here. The direct method allows the learner to successfully express themselves with only the target language, without mixing words with their native language. This though can be tricky since a lot of words cannot be used with their literal definition (lack of vocab). Wikipedia claims that the learner also develops reading and language sense though it claims that the method also ignores systematic writing and reading activities. The method though does implement quite a lot of audio-visual aids and techniques.
With the advantages, there are the disadvantages unfortunately. This method can't be used to be changed to the specifications of an individual in a large class. The flexibility of the method makes the method more suitable for individual learners. What makes that point even more severe is that this method is inconvenient for large classes. Reading, writing, and grammar are either ignored or not taught systematically.
Of course, there are tons more not listed here and can be seen in the linked sources, other websites, and of course experience.
Historically, the direct method is a language teaching method (rather than a learning method) developed by Maximilian Berlitz at the end of the 19th century. Berlitz called it the "Berlitz Method", but it is otherwise known as the direct method.
The direct method was based on the belief that total immersion would speed up the language learning process. It gave listening and speaking a higher priority than reading and writing (similar to a child's acquisition of their native language). Grammar was taught in an inductive manner and focused on the forms that are most common in the spoken language.
Classes had few students; the intensity and the small class size made the direct method unsuitable for public schools, who went back to the grammar translation method (GTM). (See Taber's "A Brief History of ESL Instruction".) According to Mehmet Demirezen, the popularity of the direct method had declined by the 1920s due to criticism from the Reform Movement.
Advantages of the direct method:
- The learner receives more input in the target language than when the language is instructed in their native language.
- The learner creates direct associations between words in the foreign language and concepts in their mind, actions, situations, etc., instead of indirect connections via words in their native language. (This can be important when words in the target language are difficult to map to words in the native language.)
- Because of the above, the learner is encouraged much earlier to think in the target language (which may reduce interference from the native language, although no language teaching method prevents it entirely).
- Classes have few students, so the intensity of the learning process can be increased.
Disadvantages of the direct method:
- Activities are much more teacher-guided than in certain other methods that allow, e.g., peer teaching/peer learning. (See Naim: "Direct Method vs Communicative Language Teaching".).
- There is no emphasis on authentic materials. (Cf. Naim.)
- Some learners may not like the delay in learning to read and write, e.g. if they need to learn the language for professional reasons.
- The direct method requires a high level of language competence in teachers, since they are not supposed to compensate lower language skills by relying on their native language.
- The small class size made the direct method unsuitable for public education.
- At the beginning, the learner may feel overwhelmed by the new language, and certain emotional states (anxiousness, frustration, anger, ...) can have a negative effect on learning. The direct method did not deal with these psychological aspects (cf. Naim), unlike some other methods such as Suggestopedia.
- Some concepts are difficult to explain without recourse to the learner's native language. (However, if an occasional translations is allowed, this need not be a disadvantage of the direct method.)
- When there is a big distance between the learner's native language and the target language, certain aspects are difficult to explain without using the learner's native language. This is especially the case for grammar and certain communicative and cultural subtleties. At the early stages, the learner simply lacks the vocabulary to understand these aspects when they are explained in the foreign language.
Finally, I would like to quote from Bob Adamson's chapter "Fashions in Language Teaching Methodology" (in: Davies, Alan; Elders, Catherine, eds.: The Handbook of Applied Linguistics. Blackwell Publishing, 2004.) Adamson quotes another researcher (Nunan), who says that there is "little evidence that methods are realised at the level of classroom action in the ways intended by their creators". Adamson adds: "The complexity of classroom realities virtually renders invalid any “one-size-fitsall” approach to method selection."
(Note: I did not rely on Wikipedia for the list of advantages and disadvantages.)