We just got back from a one week stay in January. We were two adults (both in our 30's).
I bought the Passporter Guide and the Unofficial Guide. Both of which I used endlessly in planning, while we were there, and since getting back - for journalling in scrapbooks!
Both cover to some degree information for couples looking for something romantic, or parents wanting some childfree time.
I borrowed a WDW for Couples books from the library, to see if I really wanted it before I spent the money on a 3rd guide book. I didn't really find the information any better or different to the two books I already had.
So after all that, my short answer would be - no, not necessary.