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.