iOS8 extension needs own provisioning profile?

It would seem that Xcode would prefer it that way, however there's nothing stopping you from using a wildcard profile until you want to do an Adhoc or Release to the App Store. More to the point of your question though, for every target that you have, you need a profile and bundle ID. So yes, you "need" another provisioning profile for your Extension, although I expect Apple to streamline this process.

All in all, it's probably best to just use a WildCard dev profile until you need a provisioning profile. Remember this is all a beta, and they're still working on pretty much everything.

Extensions are separate targets from the host app. You have to treat them as though they are separate apps with different bundle identifiers and different provisioning profiles.

It's best if you just create them during testing instead of using wildcards because you may need to test some capabilities that you enabled, for example App Group so that your extension and host app can access the same container.

Note that if you want to have any capabilities enabled for your host app and extensions, you have to enable them individually for all of them.

I've located this documentation pertaining to App Extensions. If you read the bottom of page 20 if seems that using the same distribution certificate is actually suggested. I've gone ahead and changed my bundle identifier for the Today Extension to extend my applications. For example, com.DeveloperName.AppName.TodayExtension, created an App ID in the Dev Center to correspond with that, and created a distribution profile for it. My app +Quotes has successfully been updated by following these steps. I have not experienced any issues with the app or extension, but Xcode does seem to get a little confused when trying to auto assign the correct provisioning profiles for my extension now. Selecting them manually solves this.