First a caveat, I dont teach right now, and havent formally taught in several years, even then it was a GTA sort of experience not a lecture hall. I'm also actively looking or jobs with a teaching component, so its not like I have a totally unbiased perspective here.
However, a recent trip forced me to think (in slightly more coherent terms) about the role of teaching in a scientists life. In my field at least, there seems to be, at times, a selection against competent teachers ever getting in front of undergrads. The people who are excellent communicators, listeners, original thinkers, have the best breadth of knowledge about their topic and are the hardest workers (ie the best scientists) are often directly or indirectly encouraged to do only science. These skills are of course the things that make the best educators.
Now I used to think primarily about a teaching load as what that experience could do for me. I could teach a course relevant to my field of interest and I could harvest the fact-questioning and creativity of the students to further my own research program. That sounds great for me, but maybe not so great for them. There is another perspective though, which I think might be disregarded quite a bit. The public has funded my training as a scientist (through both University funding and more directly through NSERC in Canada and NIH/ADA in the States) and its my responsibility to pay that investment back.
Its great to think that my research someday may cure some devastating disease, or will directly improve the human condition. At the end of a scientific career, even a very good one, how many people can point to a specific direct impact. Most researchers (especially basic scientists) can improve a field, and provide insights that then may go on to help people more directly but most of us are not inclined, or capable to directly impact the public.
How then can we have an impact and pay back the public investment in our careers. It seems to me that a teaching responsibility might be the most efficient way of repaying that investment. An inspiring teacher, who can convey both the benefits of the scientific enterprise and point out how it improves the human condition can have an incredible impact on an undergraduate's future opinions of science. If that student does not end up as a scientist (as most won't) that experience will carry with them to wherever they go and whatever they do. If that student has learned an appreciation for science (or a more specific field in science) then they will carry that to their colleagues and their friends and their families and will spread that appreciation. In the end that kind of viral spreading will help pay back that investment in me much more efficiently than anything I am likely to directly accomplish through research.