I think you're glossing over several things in this thread, or else just didn't read. Multiple points have been made that are quite valid:
#1. A res does not provide cooler temps unless specifically designed to do so
#2. A res cuts down on your total flow capacity
#3. A res that's misshapen or too small can actually cause bubbles to continue flowing rather than catch them
#4. The above two items can lead to a res causing MORE heat in your water (pump cavitating due to air bubbles, pump creating more heat due to increased head, rad and block becoming less efficient due to less flow)
There are still pro's to consider, but they come out relatively even:
#1. A properly sized / shaped res can help you bleed the system
#2. A properly sized and placed res can also help keep water continuously in the pump
#3. A very well designed res can assist in lowering temps slightly
#4. A pretty res is nice to look at
A tee gets around basically ALL of the cons, and the only pro you're missing is the prettiness.