Persistent notifications take up less space in Android O which is great if you value having a more organized and streamlined feel to your phone's user interface. While the notification section might seem like the last place you would care about tidiness, having a neater setup can actually be immensely helpful because in this particular situation, with Android O allowing persistent notifications to be more compact it means there's more room for more notifications, whether they're also persistent or not. This means there will be less of a need to always clear things out and the notification tray shouldn't feel as cluttered when you have a lot of notifications that have stacked up.
With these new changes things look mostly the same, and persistent notifications stay at the very bottom of the tray just as they do in Nougat and prior versions of Android, but they are now slimmer and are only one line of text instead of the fatter notifications which take up three lines of text. The slimmer notifications don't generally show all of the text that they would normally, but this doesn't mean you'll have to miss out on any of the detailed information that they would contain as you can simply expand them and when you're done they'll go back to their collapsed state to save space like before.
Since this is just the first Developer Preview of Android O and there are three more in the works before finally getting to a stable public release of the software, there is plenty of time for Google to change something with these notifications whether it's the way that they look or the way that they function. Having said that they seem to look and function pretty nicely compared to before and there really may not be anything else that Google has to do here to improve upon them. While not likely, Google could also choose to change the notifications for other versions of Android to look and function as they do in Android O and that would probably make a bunch of users happy, but even if they don't it probably won't be long before an app is made available that will mimic the style of Android O persistent notifications, allowing just about anyone to have them.