That's a good point. In fact, you can't cross borders in the Schengen area as an EU citizen unless you show some proof that you are, indeed, an EU citizen - for example, a passport.

Although other documentation, like national ID cards will do, they can cause trouble. I don't know if it's official policy or what, but I travel by land and sea around the EU very often and I've had problems with over-zealous border officials many a time.

For instance, I've been delayed twice at the French border from and to the UK by French officials, who pretended they couldn't determine whether my Greek national ID card is valid (allegedly, because it doesn't have an expiration date; Greek ID cards currently don't) and kept repeating that "if you had a passport, things would have been so much easier, now we have to call a specialist to examine this document" wink-wink, nudge-nudge.

[The funny thing is, I was travelling with my cousin's ID card by mistake, but they didn't realise because we look very much like each other- "specialist" guy, my arse]

So while officially you can travel without a passport in the Schengen area, in practice, you better have some ID.