Depending on the mission there's no doubt other alignments that would give the gravity assist.
To put it into perspective, Voyager 1 has a current speed of 17 kilometers per second. So the Dawn craft has most of that trip covered without gravity assists if it wanted. AFAIK there's nothing stopping us from loading up some craft with more fuel and larger ion engines except for launch weight. And even then, if we mastered orbital refueling we could surpass that.
As it stands, the craft can carry approximately the same amount of Delta-V that the rocket that got it into space. Equivalent to tons of fuel from chemical rockets.
And the Earth has 30 km/s, yet we don't get Voyager "for free" and then some just from LEO. You can't compare speeds in different parts of the gravity well against each other because then you're ignoring the dv required to get from here to there.