Flags are pretty much the same across all SE sites. Even if you are not knowledgeable on a particular site (say, Cooking SE), you're probably still capable of telling whether something is off-topic, inappropriate, spam, etc.
To flag, you need to be logged in that site. To log in, you need to register.
I suggest that if the system knows that you're logged in as a trusted user in another SE site, then it should let you flag posts regardless or being registered/logged in this particular site.
I know that registering is very quick if you have an account in another SE site. It even gives you bonus reputation. But I still wouldn't want to add another site just for the sake of flagging something. I could delete my account, yes, but that's kind of annoying.
And of course, I could just leave it be and let someone from that site handle/flag it. But hey, the more help the better, no? :]