Magilla, this is going to shock you, but the interspersing of clips throughout the show has been going on WAY longer than you remember.
Like you, I think of the tradition as being that, just prior to the presentation of the best picture prize, a clip was shown from each of the (then-)five nominated films. Back at the point I joined the audience (early 60s), film clips showing up on TV were a real rarity, so this was often the only chance audiences had to get a glimpse of some films. (There were no other clips shown throughout the show -- i.e., for the actors. It was strictly best picture.)
That changed in -- brace yourself -- 1966! For whatever reason, the powers that be decided that year to intersperse the best film clips throughout the evening, rather than save them for the finale. Back then -- in my youth -- I used to type up an Oscar post-mortem the day after. I just dug up my sheet for 1966's awards, and see that I complained mightily about this break from tradition. But apparently Academy folk were happy with it, because they repeated it for at least the following year. They did revert back to "save for the end" at some point in the 70s, but the idea kept returning, and by the 80s, at least, the interspersing had become the norm.
And two things made it worse: first, instead of showing clips, which were at least interesting (you could try to guess which clip each film would choose to illustrate itself), studios started simply showing their trailers, which were over-exposed and dull to watch. (I'm pretty sure this is another thing we can blame on Harvey Weinstein; I believe he started the trend.) And the second thing, of course, was, when the Academy expanded the number of nominated films, the showing of the clips began to consume way more show time.
Oddly, of late, they've occasionally gone back to the old way: as noted, in 2010 they did one big collage of the 10 nominees, and, year before last, all the clips were saved for the end (this was forgotten in the wake of the La La/Moonlight craziness). But who knows what to expect this year, when every decision the producers make seems to be the wrong one.