I'm not saying that ion propulsion is risky. DS-1 proved it's very capable. Ion propulsion in one form or another has been around for decades but it took the DS-1 team to risk their mission on it.

I'm saying that due to the need for a lot of electricity, ion propulsion to the outer planets is tough unless you're talking about nuclear power. It's a different proposition altogether if you're talking about the inner planets. That's why I was a little surprised to see that MESSENGER doesn't use it.

Risk avoidance is a big part of project management. I can understand how some managers wouldn't want to bet their missions (and budgets) on a new technology when there's enough risk inherent in the mission itself. There are some pretty tough budget constraints on Discovery class missions. The total cost including R&D, launch, and operations can't exceed $299 million.

Ion propulsion with its increased electrical demands would probably add a few million to the space probe's cost. The choice then is what do you cut in order to allow for ion propulsion. Ion propulsion on a long mission might pay for itself if it reduced the mission time by a year or two, with the savings coming from mission team personnel costs. Good geeks don't come cheap, and JPL has some of the best geeks around.