You also need a level in a warrior class to qualify for the PrC, putting you at 2 levels below a dedicated spellcaster.
Fighter's BAB is nice, but combined with the spellcaster levels you needed to qualify, you're still a few points short of a dedicated warrior. You probably won't be wearing armor, and will need magic items and spells to boost AC, and you don't have the hit points to stay too long in combat.
Overall, seems a pretty reasonable trade-off. It may be a bit more balanced than the Mystic Theurge or Arcane Trickster.
BTW, the Eldritch Knight is not based on the Spellsword. They fill similar niches, though (as fighter/wizard PrCs). The Eldritch Knight is better at fighting and spellcasting, but the Spellsword gets some bonus powers.