if you not maximizing, it is pefectly fine to learn any language. It is fun and not totally useless exercice. Hey, I tried to learn Latin, what can I say? :)
But if you maximize you need to decide where you plan to travel and use the new language. I assume people learning these pan-slavic artificial languages want to travel salic states in Europe. But once you cross a border from Polan to Germany, from Ukraine to Romania, from Solovakia to Hungary, from Czech republic to Austria, from Belarus to Lithuania, your laguage skills will become usless. My advice to learn a language which you can use over all of Europe, not just slavic countries.
But if you maximize you need to decide where you plan to travel and use the new language. I assume people learning these pan-slavic artificial languages want to travel salic states in Europe. But once you cross a border from Polan to Germany, from Ukraine to Romania, from Solovakia to Hungary, from Czech republic to Austria, from Belarus to Lithuania, your laguage skills will become usless. My advice to learn a language which you can use over all of Europe, not just slavic countries.