In a passage of his book ‘Fluid concepts and creative analogies,’ the famous A.I. scientist Douglas R. Hofstadter ridiculed small, pocket bilingual dictionaries while praising thick multilingual thesauri. Although I see his point, I cannot agree with him on this matter (to be sure, the book is fantastic). Indeed, it would be unreasonable to pick up a pocket dictionary of, say, English and expect to find a comprehensive coverage of the English lexicon, let alone an introduction to the language. However, dismissing pocket dictionaries altogether does not seem reasonable. As in the case of any other resources, I believe it depends on the use one makes of them.
It is just wrong to assume that every person who buys a dictionary aims to become a fluent speaker of the language. In my experience as a private language tutor, there are many reasons why people may want to become familiar with a language. Some prospective learners may have family in a country where the language is spoken, some other may just enjoy going there on holiday, yet other may have a passion associated to that country (e.g., in the case of the UK, it is often pop and rock music). In all these cases, they have no ambition to attain a high proficiency level; as a consequence, they will not spend a lot of money on thick dictionaries and grammar books, and/or compelling and expensive courses. As a private tutor, it is crucial for me to always tailor my classes to the individual needs of every single student. The choice of the resources to be used must be maximally functional to reach the goals set at the beginning of the course, which sometimes can be very ambitious (as in the case of an absolute beginner who is planning to move abroad within six months) but are often more humble (as in the case of a learner who enjoys visiting a country for couple of weeks a year).
Of course, a professional linguist and/or language teacher will need many advanced resources. Indeed, we often spend a long time in libraries scrutinizing dictionaries, grammar books, textbooks, exercise books, collocation dictionaries, idiom dictionaries, urban dictionaries, lists of common mistakes, and we do it mostly out of interest. The linguist’s passion, though, is often not the language learner’s. The vast majority of learners will content themselves with carrying out relatively down-to-earth conversations with both native and non-native speakers of the language. Each case will require a careful selection of appropriate tools, possibly a combination of paper and electronic resources. The only really bad resources are those which contain factually wrong information (or are poorly written and/or organized). As for the rest, it depends on the use teacher and learner together make of them.