I think there is a bit of a difference between being an average to below-average tackler at corner and an adequate tackler at FS. Newman got trucked by Aaron Rodgers trying to deliver a kill shot on the sideline. There aren't many free safeties in the NFL that wouldn't light Rodgers up like a Christmas tree given the same opportunity. Newman could be given the job of free safety, and I can drive with my feet... it doesn't mean I should. With his fragility, asking him to gain ten pounds (which would be necessary) and use his body as a battering-ram would be asking for an IR spot. He was tackled by a running back after an interception and had to sit out several plays.
I do like his ability to cover, but there are few instances where a CB moves to FS in the NFL. If it is felt that their natural weight is in the 210 pound range and they are heavy hitters, it is feasible. For example, Malcolm Jenkins with the Saints was a converted corner, but he was at 204 at Ohio State, and had a 4.5 40 time. He had instincts and liked to hit, so he was converted to FS.
This is what real free safeties look like. I know that Cowboy fans haven't seen one in a while, but here are a few clips to give you an idea of what is required for the position. Do you think Terence Newman can do this?
http://www.youtube.com/watch?v=vqFa-RDDi6U
Or this?
http://www.youtube.com/watch?v=qwyHu...eature=related
Or this?
http://www.youtube.com/watch?v=X8BEd...eature=related
Poor Percy Harvin. Those are free safeties. And, outside of the hit against an unsuspecting Roddy White last season, I can't think of many times T-New has knocked the crap out of a wideout, preventing him from making a catch. He can, however, cover wideouts and use his speed. That's what corners do.
I have no idea what you are basing your view that Newman can come up and hit receivers or running backs on, to be completely honest. The same arguments were made for Anthony Henry, but the conversion from CB to FS isn't simply a decision to move a guy over a few feet on the football field. And those tackles (or attempted tackles) he made while "trying to hurt people"... he will be asked to do that at least 15 times per game at safety because that's what the position demands of you. And he is made of glass, as his career has shown. He may play though most of his injuries, but having a FS who shies away from contact because he's dinged up will get you beat every Sunday.