With respect I think this is a terrible rule. It's not exactly good to do a check on something and then realized that it's pointless and kind of gets ignored, due to the fact that while you're checking something a member of the team can swoop in and undo all your work. At Nobody is likely going to go through an amateur check immediately after a full one is done, which just means that it would get ignored. If you really think that an amcheck slows down the entire process that much, then just ignore somebody's placeholder and do a check anyway. That still leaves room for people to placeholder and those that would want to look over someone's amcheck before doing their own.
This just seems to discourage people from checking. I know that not every amchecker makes correct changes every time, but most of the time a lot of them are useful and take a bit of work out for a real GP checker to do. If the deal is about trying to speed up the process, in these rare situations you can always just blow past an amcheck anyway. I don't think I've ever been in a situation where an amcheck takes that long to do where it slows down the entire process. It's not like the GP team gets things through at lightning speed anyway, and we certainly would never be able to get an analysis done faster than an amcheck could be done. If I saw somebody's placeholder and saw they were offline, I'd probably ignore it, but I think that most of the time this wouldn't happen. I've always respected amchecker's placeholders, and I know everyone respected mine while I was still an amchecker.