Usually in science it's not all about being locked up in an office and learning and to acquire as much knowledge as possible, but also about meeting and discussing.
Let us assume we have a textbook that relates perfectly to the student's personality, his background, his prior knowledge, and is perfectly aligned with the course's aims.
Let us further assume that the textbook is accurate, up-to-date, stimulates reflection of the material, and relates the lessons learned to the student's past and future (professional) life.
Let us assume we have a lecture that achieves the same.
A good lecture will add to what a good textbook provides:
- a platform for interaction with an expert in the field,
- a platform for social/peer interaction,
- a platform for networking,
- a platform for scientic discourse on a small scale.
Not considering the likelihood of establishing either a textbook or a lecture that actually meets above assumptions, I think the "social" aspects of a lecture outweigh the theoretical throughput of information one can feed into a student's head.
Yes, I think colloquia and seminars do even better serve this purpose than a lecture.
Yes, I admit, the number of lectures not worth attending is probably larger than the number of textbooks not worth working through. But this strongly depends on the place you study at. If you're not satisfied at your current place you might try for a change.