While I agree this is a team game and you should go for balance, I don't agree that you should allow the past to dictate the present. I don't think prior bad investments at a position justify not making a present good investment "if" it's the best option at the time.
Going by your argument, the Lions shouldn't have taken Calvin Johnson and that would've been a huge mistake. If a player is a bust there is no "coaching him up." You simply have to live with the mistake and try to do better with the next decision.
If the Dolphins are in a position where the best player available is a WR, and there is little doubt that he is substantially better than the available o-linemen, TEs, and linebackers, then I think they should take him regardless of what they already have on the roster. The draft is less about immediately improving your team and more about the long term.
Now obviously that scenario is not exactly likely, and I too prefer a balanced football team but I just wanted to make the point that focusing too much on balance can also be detrimental if it sacrifices overall talent.
While I agree that passing on Calvin Johnson would have been a mistake. This is hindsight. And keep in mind, Calvin Johnson has yet to get them a Super Bowl. Even though being arguably the best at his craft over the time he has been in the league.
And financially, yes, you do have to live with the mistake. While we only have a finite cap and portions of salaries are gauranteed, you can't just cut a guy after one year based on crafty structuring. It may be palatable the first year, but a few years later your still paying for a guy who isn't on your roster. And while you may be able to draft his cheaper replacement, you've still used a pick (resource) and spent more money fixing a problem.
A fiscally responsible GM will not continue to reinvest resources into a position (except QB) year after year and produce champions. There comes a point where you admit the coaches cannot develop talent and/or the GM cannot pick talent. At some point, someone is losing a job. Since Matt Millen has left town, they have drafted in the trenches on both sides and Matthew Stafford. Calvin Johnson was a Millen pick.
But if you look at their first round picks after Millen, 1) Stafford, 2) Pettigrew, 3) Suh, 4) Fairley and 5) Reiff, you can see the true core of their team has been built from the inside out.
Particularly regarding this thread's topic, the reason we do NOT draft a WR is because champions are not built from the outside in. And continuing to pursue this philosophy in back to back years with FA and draft picks is foolish. When you build inside out, you make the rest of the team better.
Had this not been solely based on the WR position, I would have agreed with you more.