It has been my experience that Spanish, for the most part, is still taught as a 'foreign' language in the U.S.-an exotic tongue far removed from our everyday lives that only exists as a truly living language, instead of a second language with millions of native speakers right here in our own country who use it not only for private, personal communication but also in business and in some areas, for public or political matters.
There are some advantages to learning Spanish as a foreign language in the U.S. It's fairly common to be exposed to a range of regional variations of different vocabulary, usually for things like food, clothing or common colloquial expressions, while focusing on the most widely used words. This is a good thing, at least in theory, since it would allow us to understand or speak to people from different countries and not limit us to just one and that is quite practical for living in the US where Spanish-speakers come from a variety of places.
Spanish is a rich language and certainly beginning students should be aware of some of these differences in vocabulary.
For a more advanced student, however, this can become a double-edged sword. He continues to grow his repertoire of regional varieties through study, travel and contact with native speakers, but that lexicon is possibly a hodgepodge of vocabulary, mixing words from different countries in one conversation This might sound strange or confusing depending on whom you are talking with.
I think it's important to know how people say thing in different places but depending on where you live, knowing a particular regional variety as well, can be just as valuable particularly if you want to be involved in a local Hispanic/ Latino community. For example, if you live in the Southwest, Mexican colloquial expressions, slang, vocabulary, and pronunciation would be worthwhile, in the Northeast, Puerto Rican or Dominican varieties, according to the demographics of the area, and in South Florida, Cuban.
Knowing a wide variety of regional usages takes time to learn and is a commendable goal but there are some 21 countries with Spanish as an official language and it's unrealistic that anyone would learn the unique vocabulary, when it differs from most other countries, for even a handful of countries. Certainly by living there or even in an emigree community for an extended period of time, it is more probable that at least a learner can become proficient in one.
If you have been in the situation of mixing 'dialects' or are trying to specialize or diversify your Spanish, please share your stories here by leaving a comment!