Keep in mind that the Voyager probes weren't designed to go as fast as possible, they were designed to do planetary surveying while taking advantage of a rather favorable set of planetary alignments [1]. They could have been set on a much faster course if we were just concerned about getting them as far out into space as fast as possible. Voyager 1, for example, had its path altered away from Pluto to do a more in-depth survey of Titan.

Regardless, using planets to help speed things is just good sense. Think of going on a long trip in a car. You don't hook a trailer full of all the fuel you'll need to get to your destination, you stop at gas stations along the way. Using planets is a lot like that.

[1] https://en.wikipedia.org/wiki/Grand_Tour_program