They should always use "unknowns" when casting ANY horror movie. It just isn't the same when you know or recognize any of the people... not for horror, anyway.
The only exception is that no Friday/Jason movie should ever be made again without using Kane Hodder as Jason. While he wasn't the first, the role is his and should always be his for any future productions.
Jared Padalecki was basically unknown outside of Supernatural, with the exception of maybe House of Wax. Moving from TV to Film with a movie like this is pretty much routine.
Also, I don't think this was movie was ruined. In fact, outside of the original and probably part two as well, this is the best movie in the series. Rebooting Jason as a crazy guy in the woods killing kids who were stealing his weed plants, while also maintaining the background from the original, was a great take on the Jason concept, and the acting, writing and screenplay were all superior to the originals. The 2009 movie was a reboot done right.
I agree with your main points (horror movies don't NEED only unknowns), and I liked this remake well enough. Jared Padalecki was also well known from Gilmore Girls.
'They should always use "unknowns" when casting ANY horror movie. It just isn't the same when you know or recognize any of the people... not for horror, anyway.'
There have been plenty of enjoyable horror films with well-known actors.