In the many conferences I attend, it's common for me to see a 40-minute talk that would have been better with 10 minutes of the weakest material removed.
Conference organizers think they are doing speakers a favor by giving them ample of time to cover their topic, but I believe this is mistaken. It is far better to give speakers too little time than too much. The first forces them to present the essentials quickly and leaves the audience wanting more. The second makes one worry about filling time. It seems reasonable that the latter leads to talks.
When I coach speakers, my advice is almost always "get to the point faster, then stay there." Shorter slots encourage this positive behavior.
If you take this principle to the extreme, you get lightning talks; often only 5 minutes long. These are nearly always a highlight of a conference. Speakers have no time for wandering introductions or apologies, and instead dive immediately into their best stuff. Then, before you have a chance to get bored, they're forced to wrap up. If you're interested in learning more, a quick Googling does the trick. This, by the way, is what I think conference talks are best at: providing just enough information to convince you to research something more deeply.
Shorter slots help a speaker focus their material, but they have another benefit: eliminating the colossal challenge of holding an audience's attention for long periods of time.
In my experience, there is usually a minority of speakers at any given conference who can keep an audience engaged for 40 minutes. The speakers that can't are not necessarily bad, they've just been giving an incredibly hard task. Consider this: nearly every attendee has a laptop. Speakers are competing against the internet amongst a group that will switch to a browser if their tests take more than 2 seconds to run.
If I were king of the world, I'd make 30 minutes the absolute maximum talk length. New speakers start with 5-minute lightning talks until they prove they can handle more.
As always, there are exceptions to this rule: some speakers can plumb deep topics in entertaining fashion for 40 minutes. But for every speaker I've seen who can pull this off, I've watched twenty more slowly lose the room to Twitter, Hacker News, and the irresistible appeal of unread emails.