This has always struck me as a combination of the worst of both worlds. By definition, if you're just omitting words in your slides that means you're coming close to reading them. And at the same time, you've made certain that your slides are useless after the fact.
In fact, that technique just strikes me as a lazy way for professors to force students to pay attention to presentations that are apparently not otherwise interesting enough.
A comprehensive slide will make the perceived value of the lecture and the slides equivalent. Hence you will skim over a slide and say, "oh, they're talking about this thing now" and then go comatose for the next few minutes. Leaving a single fact as an open question acts to raise the perceived value of everything else.
The slides aren't useless afterwards because the version put up afterwards may contain all the words.
Saying this is lazy is like saying that giving students problem sets without giving solutions simultaneously is lazy. It's the same principle - if you give out solutions, you are asking the students to not cheat by working backwards from the answer, to have 100% motivation to understand the problems on their own. No student is always that motivated all of the time in all of their coursework, but tricks like that will induce them to the extra effort.
Teaching has both "hard" techniques and "soft" ones. Using both will speed learning.
That's why your slides should be supporting your talk. The deck shouldn't be comprehensive -- your presentation of it should be. And if "the version put up afterwards may contain all the words"--if that's all that it takes to finish off the presentation--then, again, you're taking an annoying shortcut to try and get people's attention which will only serve to piss them off.
In fact, that technique just strikes me as a lazy way for professors to force students to pay attention to presentations that are apparently not otherwise interesting enough.