In the Deathly Hallows, Yaxly grabs onto Hermione while she is disapparating, and she is forced to take him with her. He sees Grimmauld place, and because she had become one of the Secret-Keepers after dumbledore died, Yaxly knew the secret. But, he shouldn't be able to bring other death eaters in, because only a secret keeper can tell the secret, and when a secret keeper tells you, you do not become another secret keeper.
- "If Yaxly could now get inside the house, there was no way they could return. Even now, he could be bringing other Death Eaters into the house by Apparition"
- —Harry's worries about going back to Grimmauld Place.[src]
So, were Harry's worries meaningless? Yaxly should not have been able to do anything about Grimmauld place, and that is only one more Death Eater that can get in besides Snape. And couldn't they put up some extra protection like Hermione and Flitwick did on the tent and Hogwarts?