In sports it has been found that to improve in sport A it is often a good idea to practice sport B. Tennis or ping pong or gymnastics may improve strength or quickness or balance needed for ice hockey, for example.
I think the same works in language learning. If a person speaks only one language and starts to learn a second language, I believe there are many advantages to starting to learn a third language as well.
First of all I believe learning a third language will improve the language learning capability of the brain. There is no doubt that I am a better language learner now, at the age of 59, than I was as a seventeen year old in school trying to learn French. I have learned so many other languages that I am simply better trained at language learning.
Second, the more languages you becomes familiar with, the more you become independent from your native language and culture. This independence is important in terms of your attitude towards a new language and in terms of your ability to master the structure, vocabulary and pronunciation of another language.
I do not recommend studying two languages at the same time. I do, however, think it is worthwhile taking a two or three month break from your major target language (say English) to study another, say Spanish or Chinese for a few months. The goal is simply to train your mind. You would not expect to master this minor language, but rather to cross train your mind.
I am not sure this works for everyone. I m sure it works for me. I hope that in another 6-8 months we will be able to offer more languages on The Linguist so that our learners can test this theory.