That is a common debate, but the general consensus is that middle tier closers are expendable, but the top tier closers are more valuable. I would actually argue that closers are more important in this league than standard leagues because of the use of Net Saves instead of just saves.
Top tier relievers have significantly better ERA/WHIP values that will bring down those stats for your whole team. Waiver-wire closers are hit-or-miss, and can hurt you significantly in other categories. We are using Net Saves, so a bad closer blowing a save will hurt you.
The other factor is K/9. The top tier closers will bring in significantly more strikeouts than the bottom tier. Kenley Jansen for example, has a 15 K/9 this year and 14K/9 over his career, Holland too. Last year came 111 for Jansen, 103 from Holland -- that's a brick ton, especially from guys who produces ERAs in the 1.00s, and Whips in the 0.80s for the team.