The problem isn't well-roundedness, it's the halt in gain in raw power. A primary caster gains increasingly powerful one-shot kill capability as s/he gains levels, AND has a variety of tricks up his/her sleeve. A combat character like the rogue or barbarian gains steadily escalating damage capability, in addition to a better BAB, more attacks, and better ways of exploiting that damage capability. The fighter, OTOH, gains only additional tricks. Let's assume only core feats (plus Knockdown) and a human fighter. Try this progression:
L1: Combat Reflexes, Dodge, EWP: duom or spiked chain
L2: Expertise
L3: Improved Trip
L4: Mobility
L6: Spring Attack, Whirlwind Attack
L8: Knockdown
L9: Power Attack
L10: Cleave
OK, you're now done with both your power and tripping chains. Granted, you've skipped "flavor" feats, but how many people take those? Save feats would be useful, but that just takes you to 12th level. Do you now want to be an archer as well? There's probably someone in the party who already serves that function, and besides, the casters are going to be better at range anyway.
I'm not trying to suggest that the above character doesn't have plenty of options. My point is merely that specialists do best in D&D. A fighter specialist gets capped pretty soon. Sure, he can function in a variety of situations, but it's not like the above character is bad with a bow or a big sword. Moreover, he won't be getting to do anything that makes him better at his primary combat style after 10th level. The wizard, OTOH, is getting 6th-9th-level spells past this point.
This character has practically everything to gain by taking a 10-level PrC and having done with. With the above character, I'd probably skip Cleave and take Imp Critical, subsequently moving on to Weapon Master, or multiclass.