In that example when considering just the net bonus to AC, it is a bad idea. It costs +6,000 gp to increase the +1 spiked shield with +1 spikes to a +1 spiked shield with +1 defending spikes. It would cost only +3,000 gp to make it a +2 spiked shield with +1 spikes. That's why defending is really a suboptimal choice in most cases. Enhancing weaponry is twice as expensive as armor/shields. The benefit of the defending bonus is the type of bonus, which will apply in different situations that armor/shields, and if you really must have the highest AC possible, then it will stack with maximum armor/shield enhancements.