You're right there isn't any need to have the passport booklet when travelling within the EU. I have the passport card on me almost 100% of the time when I'm not at home, as it's kept in my wallet. So I use that when flying within the EU as well.
However, when I'm traveling to somewhere that has a hotel safe, or somewhere secure enough to leave valuables, I'll bring the full passport booklet in my carry-on along with me, and leave it there when I go out. That way, in the unlikely event I should lose my wallet / passport card, I still have the full booklet to travel home on, rather than having to arrange emergency travel documents for the flight back.
I haven't needed it yet, but I don't see the harm in also taking it. It's not as if it takes up an obscene amount of space in my carry-on luggage, nor am I risking losing it by carrying it on my person at all times. It's purely there for peace of mind, and a backup in the very unlikely event I need it.
