Sometimes, it is WORTHWHILE to invest in some EU players TEMPORARILY.
Specific EU teams play twice in a week. The point you have to decide is whether or not those EU players are worthwhile in the short-term over long-term NA players.
If you ever get a EU player, you just have to understand that you can probably only play him a couple times over the course of the Fantasy League. Besides, you can always trade in that EU player for a different EU player that plays two matches in the week.
Take, for exemple, Misfits in week 6. If you picked some of them up during that week, you would've nearly guarenteed your win for that week. Then, for week 7, you can easily trade in your Misfit players for Roccat or Vitality players who not only play twice in week 7, but also play against Origen (The team which gives up the most fantasy points).
Long story short, EU players are worth the investment only as short-term players. However, sometimes it is better to keep a solid-mid tier NA player over a top-tier EU player for a week. You just have to make the decision.