OK, I'm kind of blushing in front of my screen here.

What's happening is that POSIX specifies that to use the + terminator, the placeholder {} needs to be the last argument to exec -- that is, immediately before the +.

It just so happens that my examples worked because they satisfy this condition. vorbisgain '{}' +, flac -f --replay-gain '{}' +, rename 's///' '{}' +, etc.

A bug has been reported against findutils to get {} to work anywhere, but it's been closed as WONTFIX -- because the current behavior of findutils respects POSIX and changing it would mean no more POSIX conformance. See here for the bug report on Savannah.

You can work around this in the case of cp with cp -t /tmp '{}' +, but it's kind of ugly.