To be frank, this is the first chapter where I've actually given a shit about the kids. It's a necessary chapter for a multitude of reasons and, contrary to the negative coalition against it, it's actually very efficient writing that probably saved us some chapters. If you don't believe me, what did we gain from this?
Well first of all, the kids are actually characters now. That's a plus! We've had this sub-plot running for weeks now and it's about damn time there's a reason to actually give a shit about the characters that make up such an integral part of this arc. Oda made the effort to give us a more extensive backstory on them than just leaving it at Momo's. At the point before this chapter, they were still robotic. TLC made some great points in this regard.
Secondly, this empowers the Luffy vs Caesar build-up with genuine emotional conflict. Before, all we had to go on was a message. Momo said they were going to die from the drugs… and that's it. Just words. With this chapter, we can actually see the severity of those words in action. You can see that Mocha is clearly dying from the drugs. Say what you will, but actions speak much louder than words and it's a necessary point to illustrate.
And finally, as others have stated, this is the first time Chopper has ever bonded with a non-SH since his flashback in the Drum Kingdom arc. The parallel that exists here is that his father died from poison and he was unable to save him due to a lack of knowledge. These kids (and more importantly, Mocha) have been severely poisoned. Whether he succeeds or fails here, his development as a character is going to reach a new level and I'd bet money we'll get our "The Fifth" chapter.
All of this stuff are necessary components for good writing and it was all resolved in a single chapter. What people fail to understand, time and time again, is that this story is written to be a whole, not written to be as exciting as possible from chapter to chapter. In an ideal state, when the comic is complete, you would breeze through this chapter in probably one minute and continue on into the next. But you would gain all the clarity that it offers.
Now, if you're bored with it and just have no interest in the sub-plot? Whatever. But to argue that this chapter doesn't contribute to the plot is pure ignorance.