Research in the field of multimedia learning suggests that in most cases, "people learn more deeply from graphics and narration than from graphics, narration, and onscreen text". This is known as the redundancy principle.
Previous research has shown students learn better from multimedia
lessons containing graphics and narration than from graphics,
narration, and redundant on-screen text (Kalyuga, Chandler, &
Sweller, 1999, 2000, 2004; Leahy, Chandler, & Sweller, 2003;
Mayer, Heiser, & Lonn, 2001; Moreno & Mayer, 2002a, 2002b;
Mousavi, Low, & Sweller, 1995). This finding is known as the
redundancy effect (Mayer, 2001, 2005c). For example, in a study
by Moreno and Mayer (2002b), participants viewed an animation
about lightning formation. The first condition had narration accompany
the animation, whereas a second condition received
redundant on-screen text in addition to the animation and narration.
The group that received the redundant on-screen text performed
worse on subsequent retention and transfer questions than
did the group that received animation and narration; thus, a redundancy
effect was found.
Adding redundant on-screen text detracts from the learning
processes highlighted in Figure 1 because it creates extraneous
processing—such as inducing the learner to visually scan between
the caption at the bottom of the screen and the graphic and to try
to mentally reconcile the incoming spoken and verbal stream. If
the learner has to waste limited cognitive capacity on extraneous
processing, the learner will be less able to engage in the cognitive
processing needed for learning—essential and generative processing.
(also from )
Are there times when it is good to include some of the narration in a slide? Yes. For example:
The most straightforward contribution of this work is to add an important caveat to the redundancy principle “People learn more deeply from graphics and narration than from graphics, narration, and onscreen
text” (Mayer, 2005c, p. 193). In revising this statement of
the redundancy principle, we can add the following limitation:
“except when the on-screen text is short, highlights the key action
described in the narration, and is placed next to the portion of the
graphic that it describes.”
In a meta-analysis of the literature on the redundancy principle:
[A]dvantages of spoken–written presentations over spoken-only presentations were found for low prior knowledge learners, system-paced learning materials, and picture-free materials. In comparison with verbatim, spoken–written presentations, presentations displaying
key terms extracted from spoken narrations were associated with better learning outcomes and accounted for much of the advantage of spoken–written over spoken-only presentations.
You perceived the talk with redundant written and spoken text to be beneficial to you; this is consistent with the research as well:
Research on metacognition has consistently demonstrated that learners fail to endorse instructional designs that produce benefits to memory, and often prefer designs that actually impair comprehension. Unlike previous studies in which learners were only exposed to a single multimedia design, the current study used a within–subjects approach to examine whether exposure to both redundant text and non-redundant text multimedia presentations improved learners' metacognitive judgments about presentation styles that promote better understanding. A redundant text multimedia presentation containing narration paired with verbatim on–screen text (Redundant) was contrasted with two non-redundant text multimedia presentations: (1) narration paired with images and minimal text (Complementary) or (2) narration paired with minimal text (Sparse). Learners watched presentation pairs of either Redundant + Complementary, or Redundant + Sparse. Results demonstrate that Complementary and Sparse presentations produced highest overall performance on the final comprehension assessment, but the Redundant presentation produced highest perceived understanding and engagement ratings. These findings suggest that learners misperceive the benefits of redundant text, even after direct exposure to a non-redundant, effective presentation.
Note that the results above refer to multimedia presentations, i.e. those that include graphics. Presentations that do not include graphics may benefit from redundant spoken-written text (see ). But presentations that do not use graphics violate the multimedia principle, which states that in general, "people learn better from words and pictures than from words alone".
For more information on principles of multimedia learning, see The Cambridge Handbook of Multimedia Learning. If you're looking for something shorter, here is a brief summary of the major principles of multimedia learning, and here is a presentation by Richard Mayer.
 Revising the redundancy principle in multimedia learning. Mayer, Richard E.; Johnson, Cheryl I.
Journal of Educational Psychology, Vol 100(2), May 2008, 380-386. http://dx.doi.org/10.1037/0022-0622.214.171.1240
 Verbal redundancy in multimedia learning environments: A meta-analysis.
Adesope, Olusola O.; Nesbit, John C.
Journal of Educational Psychology, Vol 104(1), Feb 2012, 250-263. http://dx.doi.org/10.1037/a0026147
 Learners misperceive the benefits of redundant text in multimedia learning . Fenesi, Barbara; Kim, Joseph A. Frontiers in psychology 5, 2014. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4088922/
 Verbal redundancy in multimedia learning: When reading helps listening. Moreno, Roxana; Mayer, Richard E.
Journal of Educational Psychology, Vol 94(1), Mar 2002, 156-163. http://dx.doi.org/10.1037/0022-06126.96.36.199