The Price family is needed, and the Covenant exists, because of one thing: xenophobia. The fear of the alien, the unfamiliar, the unknown, the strange. The fear that the creature under your bed is here to eat you up, not just sneak into the living room while everyone is sleeping and watch American Idol on the DVR.
The fear of what might happen.
Even Prices get a little xenophobic, when confronted with the unfamiliar; their first response to discovering the cuckoos was "kill it with fire," not "hugs for everybody." Because some things are too alien. Some things can't be reasoned with, or worked with, or appreciated on their own merits.
As long as people are people, there will be xenophobia on all sides, and as long as there's xenophobia, we're going to need people like the Prices standing in the middle, asking everybody to just take a breath and settle the fuck down.