I'm reading a lot of scripts these days - for work and to learn for myself. We're staffing writers on three new series (can you say busy?) and so everyone's reading their sample scripts and debating who's best at what and so on.
Right now I'm reading a sample TV script for a detective show that has very high ratings. And the script is really good, written by a young, new writer. But it's hard to get through because it's so damned familiar. I'd much rather read something original that the writer made up themselves. Yes, that is much harder than writing a sample of an existing series. And yes, a sample of an existing series tells us the writer can imitate a series' "voice" (a very important trait in a TV writer). Lots of contests and workshops require you to write an existing show as well, so this sample is quite good for those purposes.
But it's so much more fun to read something new and different, and not to know exactly where the story will go or what the characters will do. You can't really surprise your reader if you write an existing show. And you can't really show off your own voice and writing chops in the same way as if you wrote something original. Even at the baby writer level, execs want to staff someone with a bit of a vision, a strong voice, and the sophistication to write their own pilot. It's a lot to ask, but it's also one of the most competitive jobs in the world.
So, when in doubt, write an original pilot spec script for TV. Your reader will thank you.