DeGette votes the right way and has gained considerable clout in Congress from her seniority. The fact that she is a work horse rather than a show horse shouldn't detract from that record...
Individual Congressional seats do matter. The most important thing in the House is which party controls Congress, as opposed to individual member legislative action, and the more Democrats there are who are liberals, like DeGette, the less the House leadership has to compromise with Blue Dogs in a conservative direction.
DeGette has been a reliable vote on choice, and has personally taken the lead on consumer safety issues, which matter, even if they aren't necessarily the highest profile issues in Congress.
Off the top let me say that I think these are all perfectly valid arguments, I'm just not personally persuaded by them. Starting at the top, I guess I am looking for a show horse. In my mind, if you represent a safe Democratic seat you have a responsibility to the caucus to lead on tough issues. There are other members of your caucus who don't have that luxury and if those in safe seats don't have enough fight in them to truly lead then that is a wasted seat in my opinion.
Individual Congressional seats do matter and I am a firm believer that the most important is the vote for Speaker of the House, because controlling the agenda means so much. But in a year where Dems are poised to lose the 45 or more seats and thus lose the majority by at least 8 seats then an individual seat matters less. I'm not advocating for a conservaive style war on moderates. You will never see me attack John Salazar or Betsy Markey. But when the opportunity arises to jettison dead weight and little to no cost to the caucus I have a hard time standing in the way.
About the best argument for DeGette's re-election is that she is a senior member of the House which means she can bring home the bacon. Hope and change it ain't.