Some colleagues and I were talking about teaching loads over dinner recently. First, let me say that I do not think that the word load implies that we find teaching a burden. It's just a term that means "how much one teaches per unit time" (term or academic year).
I hope that longtime readers have the impression that I like and value teaching (and students), as that is, in fact, how I feel about teaching (and students). That does not, however, mean that I want to teach a lot. By a lot, I mean more than 2.5 classes/year. OK, maybe 3.
If I teach a lot -- and there have been years when I taught > 3 classes -- I like teaching less because I don't have time to focus on doing it well. I don't have time to give my students the attention they need and deserve. Excessive grading erodes my mental and physical health.
If I teach more than 3 classes in an academic year, I don't have as much time for my graduate students and postdocs and undergrad research students, and it's more difficult to find time for research, papers, proposals, conferences, and all those things that are the other major component of my job. I have a very active research program, and it requires a lot of time to keep that going.
Although I am an epic multi-tasker and am happy work late into the night and on weekends, there are limits to what I can do in terms of doing both research and teaching well, and my personal limit -- given the size of my research program and group -- is about 2.5 classes/year.
Fortunately, that happens to be my average teaching load. It doesn't have to be 2.5 classes every year. It can be 3 some years and < 2.5 other years; that's fine. It's good to have flexibility.
Yes, I know that other faculty teach 4 (or more) classes in a year and also manage to get other things done (research, life), but I am not one of those people who can do all of that well.
Are my university and the people it serves getting their money's worth out of me? How would I fare in an evaluation of my usefulness to the university? That depends on the factors in the equation. I have brought in a lot of grant money, advised and graduated a lot of students, published a respectable number of papers, and received high teaching evaluations. I would fare well if those efforts are considered.
I would not fare well if the number of courses taught/year is a major factor (although my teaching load is not out of line with my department or similar departments). The low number of courses (relative to, say, humanities faculty teaching loads) might be somewhat offset by the fact that I teach some large to moderate sized courses, but there's no getting around the fact that many of us science professors teach less than our colleagues in other units of the university.
Should I teach more? Would it be better for the university overall if I taught more and did less research and advised fewer graduate students? I don't know.. that's a loaded question.