(not a mod chime-in, just my thoughts)
Using your example of the Persistence of Memory
and Immortal Coil
- most if not all of the references to Immortal Coil
will be happening within the discussion of Persistence of Memory
and that will be in review threads marked spoilers in the title, so that won't be a problem.
Bottom line, in my view, is that this is a discussion forum so we should make it as easy as possible for people to discuss things and that means that some of us (myself included because I'm not up to date) run the risk of finding out parts of plots of books we haven't read.
I don't think it's workable to ask someone to spoiler code a passing reference to a book published a couple of years ago. Also, I'd find that really annoying to read