Interchangeable in Hollywood

By the way, I saw this poster for the television series "Pretty Little Liars" yesterday when I was out at the salon getting my hair cut.  Can anyone tell these sweet young things apart?  Aside from one being a blonde, their faces are almost identical.  Before I posted this, I actually checked the series to make sure they weren't digitally animated!

I've remarked before on the lovely sameness settling in on television, but what is the business thinking behind this?  And what must it be like to be one of these interchangeables?  

4/6/11 p.s.  My husband the novelist-budding screenwriter ventures that on low-budget series aimed at certain demographics - teens, for example - producers go with actors who look very similar so that if one leaves to go do something more interesting or lucrative they can be easily replaced and the series continues without missing a beat.

I think he's got it!