UPDATE: The discussion of this topic is now covered by the thread on NULLs: Should append null fail, append BLANK! add nothing unless /ONLY?
This is for many reasons, including that any [...] now returns NULL on failure and not BLANK!.
Given that append block  will append nothing (you have to use APPEND/ONLY), and append block () will append nothing (the void evaluation product is not legal to put in blocks, so the assumption is you meant no-op), I am wondering about append block _.
block: copy [a b c d] append block any [false false false]
Do you usually want block to be [a b c d _]? Should you have to say append/only or use opt any to convert blank results to voids? The same question comes up with COMPOSE, and other operations.
I wonder about this often enough that I feel like we need a strong argument to pin it down. Is the idea of blanks being used as placeholders in blocks sacred enough that people not be forced to use /ONLY to append them? In my own experience, I probably want to append literal blocks much more often than I want to append literal blanks.
Don't know. I also feel like we need to get a pinned down answer to Splice by Default only when appending BLOCK! (e.g. not PATH!, GROUP!)