I don't think people should complain about missing an A rank. It's going to respawn in about 4 hours anyway and a bunch of them may spawn in between. If it was an S rank, I can understand.

I guess the only reason why it's called "Ninja" is because the mark died without people noticing. I know the hunt community usually keeps up a way to keep track of the time of death for the marks (In the form of a spreadsheet or website with updatable timers) so they'll know when the mark will approximately spawn again. If someone were to "Ninja" some of those mobs without informing other people, people may lose track to when those marks will spawn again. Finding marks may become a bit more difficult because nobody would be able to correctly update the timers and you'd see people running left and right in every zone to look for the marks. At least, that's as far as I can tell.