Good lord no. It was a religious symbol, a display of the legions spirit. No signifer was going to break that in combat! But then... if you're carrying one and no other weapon to hand.... Soldiers are practical people.
If Caesars troops gave ground he would often rally them in person, catching individual fugitives by the throat and forcing them round to face the enemy; even if they were panic stricken - as when one standard bearer threatened him with the sharp butt of his eagle and another, whom he tried to detain, who ran off leaving the eagle in his hand
Life of Caesar - Suetonius
Wow, revelling! If the butt of the standard was sharp that implies it definitely could double up as a weapon, however i am now going to guess that they didn't train with them or want to use them because they were a religous symbol.
vtc