From what I understand the purpose of the mission is not to blow it up "simply because we can." There is a reason for this mission. I found my information at
http://deepimpact.jpl.nasa.gov/mission/index.html
This is copied from there:
Principal Investigator, Dr. Michael A'Hearn and his science team want to find clues to the formation of the solar system and more about the structure and composition of comets by making a deep crater in Comet Tempel 1. The science objectives for the mission are:
1. Observe how the crater forms
2. Measure the crater's depth and diameter
3. Measure the composition of the interior of the crater and its ejecta
4. Determine the changes in natural outgassing produced by the impact
If all the great explorers had never wasted money sailing around the known world just because they could where would we all be? We need to expand our knowledge of the solar system, and this is one step.