A good reason to study a non-mainstream language that few other people study is that you might find job opportunities relatively easy to come by. Knowing such a language may also open up other opportunities for study, travel, friendship and business.
Many people study languages like French, German and Spanish, and there are many good reasons for doing so – they are spoken and studied in many countries, there is a wealth of literature and other culture associated with them, and there’s no shortage of job opportunities for people who speak them. On the other hand, because they’re so popular, competition for jobs using such languages can be quite intense, and knowledge of them alone is probably not sufficient. Those who speak one or more of these languages, and have others skills are likely to find it easier to find work.
Estonian, Latvian and Maltese could be considered non-mainstream languages: that is relatively few people speak them and study them, at least when compared to languages like English, French and Spanish. If you study such a language you may find it easier to get a job using them. There’s a great demand for interpreters and translators who speak the smaller European languages in EU institutions, and someone with a combination of a major language or two, plus one or two minor ones is likely to be much sought after.
In the case of languages like Welsh, Irish, Scottish Gaelic and Manx there’s an increasing demand for teachers who can teach them, and/or teach through them, and for translators.