TIL there are people who (try to) use grep for natural language.
Whatever the case, I'd rather not modify (GNU) grep for this purpose. If grep can be hacked to do this (tr '\n' ' ' | grep 'the orange menace' or something maybe only a little more sophisticated?) then more power to you. Otherwise, making a separate tool is probably better.
If all you're advocating for is allowing grep to use some other character as a delimeter, I might be able to get behind something like bash's $IFS or awk's $FS variable (maybe). But I couldn't get behind anything backwards-incompatible.
Meanwhile, PDFGREP isn't associated with any maintainers of grep, is it? I've never used it and I don't know if you're saying "because PDFGREP is good at handling natural language, grep should be too" or "grep should be good at handling natural language and PDFGREP even more so, but neither is." Either way, I don't follow how PDFGREP is relevant to discussions about grep (unless they are related, but I'd be surprised if that was the case.)
Oh, and this is 100% feature/enhancement request territory. Not a bug report in any sense.