If you are a European travelling in Europe, it is better to use your identity card rather than your passport, including when crossing borders.
As it is quite easy to make a false passport, border controls are very strict regarding this kind of ID.
- Presenting your identity card is faster
- The identity card takes less space than your passport
- Inspectors are less suspicious
However, you can keep your passport in your bag; could be nice to collect passport stamps from all European countries.
Bonus: If you loose your identity card, it is faster and less expensive to make a new one.
Tested and validated tip; including this weekend in the Eurostar.