Shield is a "defensive weapon" and yes, anyone who ever got cracked up alongside the head with one will tell you it definitely IS a weapon.
Spears are good long range, but in close fighting, they are practically useless. Also, if you use them one handed, and have a shield in your other hand (such as in a phalanx) they are very awkward to meneuver. More so if it's a long spear, and there are alot of people behind and to the sides of you.