Well, it might be that Bless This House isn't intended to be used primarily on the person's own home--as has been pointed out, if Michael Carpenter is any indication, their home's probably going to have a much higher threshold than normal. But it is useful if they're going anywhere else, and as I recall one of the things about a Knight of the Cross is that they're called away from home a lot.
So Michael Carpenter's Bless This House might not do much to bolster the threshold of his own house, but it might make a difference on those occasions he ends up at Harry's apartment, or if he's traveling somewhere. If he and Charity go somewhere, that's an almost guaranteed +4 to a threshold whatever residence they find themselves at.
Though this creates something of an odd case where if you have a whole family with the power, they'll have a higher threshold in someone else's place than their own, now that I think of it. Then again, it's probably fitting that a power based on faith and good works and all that jazz has more benefit for others than for the user.
Alternatively, would it be too out there to consider "A person with Bless This House lives here" as one of those narrative aspects that bolsters the natural threshold?