Far too many 1st rounders bust or don't live up to expectations. It is my contention that a proven player has more value than a pick. However, I also firmly believe that you build a franchise through the draft, not free agency. When you draft a player you are drafting him because he is talented, fills a need, fits your system and, as you said, is a guy you can "mold" to be the type of player you want.
This is a highly debatable subject, that's for sure.
You never saw good players in or around their primes getting traded. Only in the past four years or so have you seen these types of guys get moved around, (Portis/Bailey comes to mind), but these trades are still few and far between. I think that shows that NFL teams believe that you build through the draft and that free agency pools are deep enough for you to fill any remainig holes in your roster.
So while I said that a proven player has more value than a pick, because you know what you're getting in the proven player, versus the relative unknown of a prospect, I still think that you must build a team through the draft to be successful.
Sorry if I sound contradictory, but it's such a double-edged sword.