The defending property allows you to use your enhancement bonus from your weapon to add to your AC.
Next I will bring up the Shield Master feat.
Shield Master (Combat)
Your mastery of the shield allows you to fight with it without hindrance.Prerequisites: Improved Shield Bash, Shield Proficiency, Shield Slam, Two-Weapon Fighting, base attack bonus +11.
Benefit: You do not suffer any penalties on attack rolls made with a shield while you are wielding another weapon. Add your shield's enhancement bonus to attacks and damage rolls made with the shield as if it was a weapon enhancement bonus.
Now if I have +5 defending spikes on my shield and a +5 shield can I use the spike enhancement bonus to boost my defense by 5, while using the shield's armor bonus to replace the +5 I used for the defending ability for the spikes.
It seems to me that only the spike or shield bonus should count, not both, not that I currently see it as broken considering the investment.
All opinions(RAI and RAW) welcome.
I would think the defending shield spikes would only work when you attack with them. but I may be wrong.
I do not think the feat is written with a shield being enhanced as a weapon in mind. I'd treat the shields enhancement bonus just like I would GMW cast on a weapon, you do not get to use all your enhancement bonus for defense and have it 'replenished' by the spell. I would have you use the highest enhancement to hit and damage (shield or spikes) and substract a bonus from it up to the spikes enhancement bonus for the defensive property, I'd still let the shield get benefits for penetrating DR types for the +5 weapon ofcourse unlike the shield enhancement.