Yes, I think it is a problem, for several reasons.
Jill points out that it's not a problem if your original article is showing up higher in the search results than the copies. True enough, but you've got to keep in mind that what you are seeing in the search results isn't necessarily what other people are seeing. You might well see your own article rank well, but a user in a different country, or with a different search history, or with whatever other factors Google takes into acount, might see your page way down in the results and the offending copies at the top.
And even if you were sure that most people are seeing your article first today, that doesn't mean they always will in the future.
It's also possible that any links that your article would attract might go to the copy rather than the original. The person creating the link would probably not realise that a given article is an unauthorised copy.
And, finally, you might simply feel that you don't want to be ripped off this way. You might even want to take action for no better reason than to discourage this kind of theft.
Personally, I would go to at least a little effort to stamp on the unauthorised copies. I've often done that for my own articles, and have had a good success rate.