Less miners in the reliable currencies means easier mining for everybody else. There's no real profit in killing them.
Feathercoin is the biggest one after Litecoin. Feathercoin has 1320 MH/s. Next biggest coin is Worldcoin with 700 MH/s.
Hard to make proper comparisons, but in adjusted hashrate YaCoin is possibly bigger than FeatherCoin. This is discussed in the yacointalk forum:
I think more meaningful proof of interest might be taking the very first chart on yacexplorer.tk, and extrapolating the hash rate based on N. The current mean hashrate is 54 MH/sec. If that gets multiplied by 2^5 (the difficulty change since inception of the coin), the adjusted hashrate is 1,728 MH/sec today vs. a peak of 1,400 MH/sec during the early days when N=4. This leads me to believe that there is more CPU power being thrown at this coin than there was on day 1 when all the pump-and-dumpers thought they could capitalize.
That doesn't include Proof of Stake mining, which is active already.
http://yacointalk.com/index.php/topic,270.msg986.html#msg986