One of the reasons that I originally wanted to stray from the textbook purchased by our district was because of their vocabulary selection. I remember that in the Spanish I book, one of the terms to be learned in reference to describing people was "silla de ruedas" (which does not even make the cut in the frequency dictionary as it is not one of the 5000 most frequently used words in Spanish). Any word is a good word, I get it, but when the goal is to build fluency, I just think the word for "wheelchair" can wait. Or maybe, as in real life, we let that word come up when it is needed. Sticking to words that are the most frequently used, especially in the lower levels, is crucial to getting students to the point where they can read and listen with comprehension.
Another nice thing about this dictionary is that it has thematic lists. So, if you want to teach students some words for jobs or professions, this book will tell you that learning words like "autor", "médico", and "profesor" (the top 3 most referenced professions) will be more valuable to students comprehension than "marinero" (the 4,949th most commonly used word in Spanish).
Finally, seeing as over 20 different countries have declared Spanish as their official language, and even more as a secondary language, this tool can help you sort through the different words for "cake", for example. Do you teach "torta"? "pastel"? "bizcocho"? This book will confirm that "torta" is in fact the most common word for "cake", with pastel coming in a close second, and "bizcocho" (a word mostly used by people from Puerto Rico) not even making the list.
If you don't want to run right out and buy a frequency dictionary, I noticed that I was able to access the same frequency dictionary online (though if you are like me, there is something very loveable about a hard copy):
Spanish Frequency Dictionary
If you would like to ponder this topic more, here is a very interesting read on Lingholic's blog where he discusses the idea that you can understand a large percentage of written and spoken language with a relatively small active vocabulary - as long as that vocabulary consists of the most frequently used words! So, it makes sense to me to approach teaching vocabulary in this way if our true goal is to have students that are able to understand and communicate with others through the language.