It really depends on how you look at it.
Statistically getting games out of players drafted after the 4th round is significant as most wont ever play in the NHL.
The quality of the games (in terms of 4th line, bottom pairing etc.) will be easy to replicate in most cases however.
Take a guy like Tucker, 50 NHL games for somebody drafted in the 7th round is impressive, and I imagine he will get more over the course of his career. Could we have got those 50 games from any number of free agents? Yes.
Those later picks just don't hold much value, because of exactly that reason.
In terms of why say a Tucker makes the team as opposed to free agent xyz. It is likely to do with the deaper knowledge the team has on that player. If you have 2 players at similar abilities, you will likely go for the one the organisation has a better knowledge of, and that will be the one who was drafted. He will know the players, know the system etc.
So in terms of value a guy like Tucker will hold more value to us then the other teams in the league. The other 31 teams will have similar guys.
But then an organisation may value a free agent more if they want an outside voice or veteran leadership.
In terms of why you draft a guy like Tucker who only has 6/7 upside, well your oganisation still needs those types. Be that in prospect camps, the minors etc. How many players in the Blues organisation have played with Tucker vs say Joseph? Joseph definitely has a higher ceiling and is a better player, but both likely hold similar value to the Blues at this point.
I doubt either would return a 4th if we looked to trade them.
Its far to simplistic however to say a players value is based purely on their game performance (although obviously that is the biggest factor by a long way).