Just to take a moment for a couple examples of the quagmire that is taking real-life logic and applying it to the game:
When you use the defending enhancement, you shift the magical aptitude and 'luck' of the weapon that normally finds the weak spots in your opponents armor, to magical aptitude at parrying and being at the right place to knock your opponents weaponry away. However, your weapon is still masterfully weighed/aerodynamicly designed so you should still get the mundane bonus.
You do have to use the weapon to gain the defensive bonus. From the DMG:
Defending: A defending weapon allows the wielder to transfer some or all of the sword’s enhancement bonus to his AC as a bonus that stacks with all others. As a free action, the wielder chooses how to allocate the weapon’s enhancement bonus at the start of his turn before using the weapon, and the effect to AC lasts until his next turn.
Moderate abjuration; CL 8th; Craft Magic Arms and Armor, shield or shield of faith; Price +1 bonus.
To me, using in this case is rolling to hit something
Actually, the use of armor/shield spikes were mostly used defensively in real life, either to catch a blade as it skittered across the armor/shield, and thusly mess up proper fallow-through, or to intimidate/demoralize your opponents so they aren't using all their offensive capabilities as they wonder if this sharp spiky dude is really worth fighting.
If you were dual-wielding two weapons, with one defender, you wouldn't need to fight defensively, or go total defence to shift the defender's enhancement to AC; shield/armor/knee/elbow/shoulder/forehead/codpiece spikes are just additional weapons.
If you feel that a player is going overboard on armor spikes (no, Freud has NOTHING to say about my +5 defender codpiece spike...), I'd suggest A) cut back on how many spiked things you'd allow, 2) incorporate the characters proclivity for spikes in-game (sure he's a lawful good paladin of Pelor, but with those spikes, spines, and blades sticking out everywhere, he LOOKS like a cultist of Demogogian, jobs have been strangely just been filled, hirelings keep asking if you're feeling alright, the local tavern closes up for lunch just as you approach), and may St. Cuthbert help you if you ever get knocked prone, instead of needing squires to get you up, you need squires with prybars! ¾) you could modify the untyped AC bonus to a typed one, or go half-n-half say main and off hand the AC bonus is untyped, but for everything else it's a morale bonus to AC.