Actually, I think the eldritch knight is balanced when placed in a party.
Let's posit several possible Eldritch Knight Builds:
8th level
1a. Fighter 1/Wizard 5/Eldritch Knight 2
BAB: +5; clvl 6; Average HP 30+con bonus; Bonus Feats: 2 fighter, 1 wizard
2a. Fighter 1/Wizard 6/Eldritch Knight 1
BAB: +5; clvl 6; Average HP 29+con bouns; Bonus feats: 2 fighter, 1 wizard
3a. Fighter 1/Wizard 4/Spellsword 2/Eldritch Knight 1
BAB: +5; Clvl 5; Average hp 34+con bonus; Bonus feats: 2 fighter; Spellsword channel, Reduced Spell Failure
Fighter 8: BAB +8/+3; clvl 0; Average hp 49+con bonus; Bonus feats: 5 fighter--including weapon specialization and greater weapon focus
Wizard 8: BAB +4; Clvl 8; Average hp 22+con bonus; Bonus Feats: 1 wizard
12th level
1b. Fighter 1/Wizard 5/Eldritch Knight 6
BAB: +9; clvl 10; Average HP 44+con bonus; Bonus feats 2 fighter, 1 wizard
2b. Fighter 4/Wizard 6/Eldritch Knight 3
BAB: +9; clvl 8; Average HP 49+con bonus; Bonus feats 4 fighter (including Weapon Specialization), 1 wizard
3b. Fighter 1/Wizard 4/Spellsword 2/Eldritch Knight 5
BAB: +9; Clvl 9; Average hp: 48+con bonus; Bonus feats: 2 fighter; Spellsword channel, Reduced Spell Failure
Fighter 12: BAB +12; Clvl 0; Average hp 70.5+con bonus; Bonus feats: 7 fighter including greater weapon focus and greater weapon specialization
Wizard 12: BAB +6; Clvl 12; Average hp 32; Bonus feats: 2 wizard
And a third comparison:
16th level
1b. Fighter 1/Wizard 5/Eldritch Knight 10
BAB: +13; clvl 14; Average HP 58+con bonus; Bonus feats 2 fighter, 1 wizard
2b. Fighter 4/Wizard 6/Eldritch Knight 6
BAB: +13; clvl 12; Average HP 63+con bonus; Bonus feats 4 fighter (including Weapon Specialization), 1 wizard
3b. Fighter 1/Wizard 4/Spellsword 2/Eldritch Knight 9
BAB: +13; Clvl 13; Average hp: 62+con bonus; Bonus feats: 2 fighter; Spellsword channel, Reduced Spell Failure
Fighter 16: BAB +16; Clvl 0; Average hp 93+con bonus; Bonus feats: 9 fighter including greater weapon focus and greater weapon specialization
Wizard 16: BAB +6; Clvl 16; Average hp 42; Bonus feats: 3 wizard
Now before we go any further, we should admit that the single class fighter and wizard could easily be powered up by adding prestige classes so they represent the low end of non-multiclassing power rather than the high end.
Looking at the Eldritch Knights in comparison to the fighters:
1. Eldritch Knights 1 and 2 either have to forgo armor and rely on mage armor or live with spell failure. Therefore, their ACs will be significantly lower than the fighter's. Eldritch Knight 3 can use magical mithral chain shirts or magical leather armor without spell failure but is still unlikely to have the same AC as the fighter. (At low levels, however, they will be able to use Alter Self to take a form which grants them natural armor and somewhat offset this). In all cases, the Eldritch Knight will need to be forewarned of battle in order to compete on an even defensive footing with the fighter.
2. All Eldritch Knights are at least three points of BAB behind the single-classed fighter. They will therefore inflict significantly less damage on full round attacks. This disparity is increased by the fact that only build 2 (IMO the least efficient build) has weapon specialization.
3. All of the Eldritch Knight builds have significantly (on the order of 20-30%) fewer hit points than the fighter before con bonusses are figured in.
4. Assuming that having perfect scores in all stats is not an option (point buy or a typical set of rolls), the fighter will have significantly better physical stats than the Eldritch Knights. Typical 28 point buy stats might be (fighter/Eldritch Knight) (lvl 8) Str 18/14, Dex 13 (the fighter's wearing fullplate)/14, con 14/14, Int 11/15, Wis 10/10, Cha 8/8 with the strength disparity increasing to 20/14 at 16th level.
5. The Eldritch Knight will probably to invest money in wizard/sorceror items such as headbands of intellect, scrolls, wands, and spellbooks which will mean his equipment is not as good as the typical fighter of equal level.
6. The Eldritch Knight will have far fewer combat feats than the fighter. Thus he is unlikely to have nearly as many options as the fighter.
So, all told, the Eldritch Knight is not likely to fight as well as a real fighter. The eldritch knight can take advantage of his wizard abilities to obtain some synergies (expeditious retreat to make spring attack better, alter self for natural armor, blink or greater invis to activate expert tactician, etc) however, even a fully buffed Eldritch Knight is unlikely to be much more effective than a single classed fighter.
Compared to the single-classed wizard, the Eldritch Knight will suffer similar difficulties. He has more hit points than the wizard and his touch attack spells are more likely to hit but usually do less damage because of his lower clvl:
1. His caster level is between 2 and 4 levels lower. At best, he's casting ice storm when the wizard gets cone of cold. However, he may well be stuck casting Tenser's Transformation (which probably still doesn't make him as good as the single classed fighter unless he's got a lot of other spells running) when the single-classed wizard is casting Horrid Wilting and Mind Blank. He'll also be casting Scorching ray for 8d6 while the real wizard gets 12d6 out of it. Similarly, he's getting +1 out of his Greater Magic Weapon when the real wizard gets +2 and +3 when the real wizard gets +4.
2. He is far more vulnerable to SR. If the EK is focussed on combat (and a non-combat focussed EK just lost at least two spellcasting levels for no good reason), he is unlikely to have feats to spare for spell penetration and greater spell penetration. Combined with his caster level deficit, this means that, his spells will probably make it through SR 30-40% of the time while the real wizards spells defeat SR 60-70% of the time.
All told, the Eldritch Knight can be a competent backup spellcaster and a competent combatant, he is unlikely to outshine either a real fighter or a real wizard. In a party with both roles already filled, he can be a valuable addition. He's probably a better solo adventurer than either the fighter or the wizard. And in a small group with maybe a cleric and a rogue, he's a better third member than either a fighter or a wizard. However, a party is weakened by being forced to rely upon the EK as either a primary fighter or primrary wizard. Thus he's balanced in the D&D environment despite his obvious advantages.
Now if you want to talk about the Eldritch Knight being rather boring, you're absolutely right. The class is sexier than the spellsword simply by virtue of it being worth taking. (Taking more than 2 levels of spellsword is (IMO) not a sign of good judgement and even taking 2 is a questionable decision). However, it lacks the defined role and interesting abilities of most other prestige classes. It's much more like the fighter class than a prestige class in that regard.