Not everyone needs to participate in killing mobs if there are enough people killing them. We sit waiting for more mobs to spawn because people are killing them too fast.
I have no issue with AFKers. Better stand somewhere safe and wait for fate to pop rather than moving in circles trying to hit monsters that are dying before you can even reach them.
I don't mind low-level players joining high-level fates even though it gives them less experience points. Low-level train is a thing, people are doing it. But if their fate does not pop and a higher fate pops first, I see nothing wrong with them participating in it.