The feat should show that you are an expert in Combat.
it doesn't. It just shows you know how to fight defensively a little better.
It needs to actually do something that says "I am a professional, smart combatant. Watch this!"
The fact that it trees into all those other feats which require "expert combat" works, fluff-wise. I suppose you could make the argument for a minor bonus to CMB and/or CMD, maybe +1 to both?
I have most definitely used combat expertise and/or fighting defensively in combat. There are times when you just can't hit the bad guy (he has DR you can't cut through, or whatever) but as the party martial, it's your job to keep that enemy distracted while it wails away on you until the archer with special arrows or the alchemist or whatever can rain down enough damage to take the beastie out. Or even just hold position for a turn while everyone else runs away.
But that's actually a lie, because while I can and have fought defensively, ain't no one can afford a 13 int on point buy for a melee martial.