What credit does inclusion in "Who is Who" book add to the researchers and academicians?
They are trying to sell their books and associated vanity products. They know that their primary market for it, is people included in it.

Personally, I'd take an academic's inclusion in it to be a possible indication of gullibility.

If you've got the time, the Forbes article on the Marquis Who's Who series, "Hall of Lame", is worth a read.