In my opinion, the point of a spiked chain is to create a weapon that a single class fighter can work to master without topping out. With pretty much every other weapon, a fighter can quickly amass the related feats, making multiclassing extremely attractive. Or the fighter can expand to other areas, at the cost of having his later feats not stack. It takes a very high level fighter to have all the feats that can be used with the spiked chain effectively.
Let's take a look at core feats only:
1: Exotic Weapon, Combat Reflexes, Combat Expertise
2: Weapon Focus
3: Iron Will (yeah, this doesn't have anything to do with spiked chains, but pretty much all my fighters take it anyway)
4: Weapon Specialization
6: Improved Trip, Dodge
8: Power Attack
9: Cleave
10: Greater Weapon Focus
12: Greater Weapon Spec, Mobility
14: Spring Attack
At this point, the spiked chain fighter is mostly complete - he can make many AoO, trip, use two handed Power attack, and spring attack to trade attacks on a 2 for 1 basis. He's not really tapped out though - Imp Disarm would work with the weapon's size and disarm bonus (but the character is very effective vs humanoid opponents, so it might be a waste), Great Cleave offers superior army killing power, Whirlwind Attack can be fun, etc. But those extra feats are pretty marginal (unless I'm forgeting something major).
But say a sword and board or two handed guy will run out of decent core feats at around level 6, leaving further fighter levels only filler before he can reach the dubious heights of greater weapon focus and spec.
