In recent seasons, there has been an unending slew of police characters who come close to or do become privy to the Charmed secret. It seems like more and more of the Charmed episodes are based solely around the sisters having to hide their magic from the prying eyes of the outside world. When used as a plot tool, I don't mind it so much, as in the instances of Prue's death, and the seventh-season finale. The first few episodes of the eighth season struck me as a little cheesy, especially with the addition of the character Billie.
Ever since season 4 or 5, the main way the writers seem to introduce new characters is to come up with a new cop who's onto the Charmed One's supernatural goings-on. In earlier seasons, the threat of exposure was real, with Inspector Davidson, among others. Now, whenever the Charmed ones are about to be exposed, the intruder either asks them for help, or they befriend them, though those two are not mutually exclusive.
I believe that this whole ordeal may have begun around the time that the FBI witch hunter attacked the sisters in the fourth season (if I'm not mistaken). Since then, there has been an endless line of police characters waiting to expose the sisters' secret.
I think a significant amount of the show's appeal comes from the viewer "sharing" a secret that few others do (aside from the milllions of other viewers across the country) and the more characters that are privy to the knowledge, the less of a secret it becomes. It's then no longer exiting to hope that the sisters defeat the demon and manage to go undetected, because sooner or later, they'll just befriend another police officer, FBI agent, or other official authority, or they'll be conveniently caught in the crossfire and killed by a demon.
Bring back the old days of Charmed, or I expect that the viewing audience will shrink drastically for coming episodes or future seasons, regardless of the attempt to attract a younger audience by adding younger characters (Billie stands on point). By the way, to any WB executives who may lurk in these forums, I think any of the four sisters (Shannen Doherty included, of course) have a much greater appeal (sex, or otherwise) than "Billie", and I'm in your lowest target age-group (I'm 18 years old after all). I mean, Holly-Marie Combs is the oldest (at least on the show) and she's only 13 or 14 years older than I am, right? I still think she's HOT.