I think the proper way to use FA is to build a floor for a certain unit. Given that Vets are usually more predictable than draft picks, but cost more, its only logical to use them as guys that you put somewhere on your team and tell yourself: "Worst cast scenario, he's the guy." That plan leads you to get 2nd-3rd tier guys that you'll probably get close to market value for and enables you to properly evaluate worst case scenarios.
The draft is where you should swing for the fences, this is where you build your ceiling. The risk is very limited, at least if you draft logically, because these rookies contracts afford you alot of leeway. ie. Its very hard to come out of the 2nd round and overpay for a WR. That contract is still a discount if the guy ends up being a backup.
If you can get TY Hilton without overpaying him, I'd say its a pretty nice floor to have when guys like Parker or Williams get hurt, or when the rookie we got arent ready to really bail you out in crucial situations. Again, this is all assuming Hilton can be had on a very reasonable contract.