First off, the campaign is a very low-magic, semi-historical setting.
We're currently in about 40 BC, trying to organise the various germanic(?) tribes to stop fighting eachother and stand up against the roman empire.
We have little to no magic items (unless I'm mistaken, the only magic item is a single +1 sword with the ghost touch ability).
The lack of magic items is deliberate, as we try to keep from using 'high-magic' items and spells. (example: lightning storm is OK, lightning bolt is not)
So, no animated shields. Also, the attack bonusses from opponents rise with the number of class levels they have (as do ours) while we have virtually no means to increase our AC.
Would you be surprised if I told you that your campaign is drastically different from what the rules were written to support and thus anyone replying to this thread would not assume the drastic level of low magic the DM instituted? Obviously, the particular campaign changes things a lot. I still think Two Weapon Defense is a poor feat, though. Maybe not "worse than Toughness," as I'd classify it in a regular magic items allowed game, but certainly on the very low power end still.
Second, the guy looking to take the feat is fighting with a quarterstaff, and regularly uses the fighting defensively option to increase his AC.
He already has Two Weapon Defense. Increasing his AC with this feat seems like a good option to me. On the other hand, I haven't looked at Improved Buckler Defense (were is it from?) although no-one has ever used bucklers in campaigns I've played in or DMed. A bit of an oversight, no doubt, but there you have it.
1. Improved Buckler Defense is from Complete Warrior. As for "is it better?" In a game with magic items, absolutely. Without the ability to enhance the buckler, maybe not. Since then it's just +1 AC and causing you to take -1 on your off-hand attacks. Now, with PHB 2's feat for +1 AC with a particular shield (Shield Focus iirc) and the feat that follows it that lets you use your shield bonus for touch AC and as a bonus to resist combat maneuvers like grapple (name escapes me, sorry)...Improved Buckler Defense could theoretically still be the better option, even without the prospect of enhancing the buckler, but the TWD feat chain might actually edge it out power-wise.
2. If he uses a Quarterstaff and frequently fights defensively, PLEASE tell me he grabbed the Quickstaff weapon style feat (also in Complete Warrior)! It's +3 AC for basically just doing what he's doing already.
Last but not least: why, objectively, is Improved Two Weapon Defense a poor feat? It increases AC, more so even for someone who uses Fighting Defensively. That may be a sub-optimal way of combat (no idea actually), but how does that make the feat 'poor'?
Unless there is a significantly better feat to take to improve your AC?
Yes, in a regular campaign with magic items, a buckler can be cheaply enhanced to provide more benefit with Imp. Buckler Defense, ring of Force Shield is affordable after a while, and later still animated shield is just plain better in terms of both the AC bonus and the "cost" (unless you really find yourself with more feats than you know what to do with, an issue i've NEVER had, spending a portion of your gp on an animated shield will inevitably become the more efficient option at some point, since wealth by level is not even linear, nevermind the fact it is on continuous increase).
Even in your particular game, consider Improved Natural Armor. Can be taken repeatedly for lots of AC if you wanted. And adds to AC in all the same situations a shield bonus would, for the same amount. No pre-reqs. Granted, it doesn't give double bonus fighting defensively, but it is also a constant benefit independent of whether you get disarmed or what have you. I'd call it about equal to TWD, and is much easier to obtain.
That's core-only. The more splats allowed in your game, the more alternative feats to TWD I could probably name.
EDIT: My own houserules messed up my reply there. Imp. Natural Armor does require you to have natural armor. In every group I've ever been in, we just extrapolated from the barksin spell text about creatures without natural armor having natural armor +0 and allowed PCs to take the feat. By a RAW reading, you can't.