First of all, we have to realize that these terms are not at all long when compared to federal judges. Federal judges serve for life. State judges’ terms are only long in comparison to those of officials in the legislative and executive branches.
The main benefit of these relatively long terms is that they give judges somewhat more independence. Judges are supposed to base their rulings on the law and not on political needs. If a judge does not have to run for office for another 6 or even 10 years, that judge can rule based on his or her legal judgments, not based on what he or she thinks is most likely to lead to reelection. Long terms, then, lead to more independence.
Longer terms look bad, however, if you believe that judges should be subject to the will of the people. If you believe that judges should do what the people want, then you will want them to be elected more often. You will want this because you will want more chances to vote them out if they are ruling in ways that you think are wrong.
Thus, this issue comes down to a balance between the idea that judges should be independent and the idea that they should have to answer to the people.