Why aren't space probes like New Horizons sped up in space? So the New Horizons project is sending a probe out to image Pluto and Charon.  As they're pretty far away, it will take the New Horizons probe nearly 10 years to get there (in 2015, having been launched in 2006).  It even received a 'slingshot' speed increase from Jupiter's gravity when it passed Jupiter, increasing from 23 km/s to 27 km/s in speed.
However, why not use an ion drive or something to propel the probe even faster towards Pluto, now that it's in the vacuum of space?  You could then use reverse thrust when it got near to Pluto to slow down enough to take good shots of Pluto.  Thrusting towards Pluto could result in getting there much quicker, couldn't it?
 A: I'm not in spacecraft nor mission design, but unfortunately, one of the driving forces behind mission profiles is cost, and another is safety of the craft (so you don't lose your investment). It is significantly cheaper to use less fuel. It is significantly less risky to have fewer maneuvers. Combine those two factors, and you have less fuel for speeding up and slowing down, and less inclination to do it and risk blowing your engine off the craft.
On a side note, ion engines are very light, but they are very slow because they work exactly by Newton's Laws of Motion where they emit a very very little bit of material (albeit at a high velocity) to propel the craft. If memory serves, and it does, the SMART-1 craft took about 14 months to reach lunar orbit by using a very gradual looping trajectory with its ion engine. Compare that with the Apollo missions that took three days.
A: There is a paper which actually discusses in great detail the history of the selection process, which can be found at the SWRI site, at least for the New Horizon mission opportunity. Let me sum it down as below.
There were 3 majorly competing proposals for the Pluto Kuiper Belt (PKB) mission. One of these included an ion engine, which was not selected. It is possible that such a mission could have reached Pluto even faster, but it does take up valuable mass. The entire spacecraft only has 400 Kg of mass, and even a modest sized ion engine with a tank would eat into that budget significantly. 
The current record for speed change is held by the Dawn mission. I don't have an exact figure, but this article says it's expected total velocity change is 38,620 km/hour. This article states that New Horizons is 57600 km/hour. I can't find the velocity of Dawn as it escaped Earth, but it was definitely less than NH.
For all of this great change that Dawn had, it requires significant extra mass, as can further be seen here. Dawn was one of the heaviest spacecraft ever launched. 
The time from announcement to launch was also an extra year. The announcement for Dawn was on Dec 21, 2001, launch was September 27, 2007. New Horizons was announce in 2001, launched on January 19, 2006, thus it had 18 months less. The deadline to launch New Horizon's was very firm as well.
So, putting all of this together, it is possible that a mission to Pluto would be faster than a conventional mission, although not extensively faster, but it would have cost more, and probably taken longer to launch, and added extra risk. But with Dawn's success, missions like this will be more possible in the future.
