No. It's quite suspect and I think that's the general feeling here but it still gets used often because of the summary nature of this measure. It also does not need to be precise in this context. Look down the list any year and the ranking by WAR looks pretty reasonable. In other words, it's hard to justify the gigantic cost and risk associated with this type of contract (see Heyward) for a player that has not ranked as elite in 4 years.
It appears to me that one great year gives players a higher profile than 1 great year of performance deserves. Compare Harper's first six years to Albert Pujlos. It's not even close
The differences are Jason Heyward had a .784 OPS entering free agency (Harper is at .900) and Pujols didn't reach free agency until he was entering his age 31 season (Harper is entering his age 26 season).
And while Harper's only had one insane MVP season, he's had several great seasons now. He's 26 and one of the best offensive weapons in baseball, he's going to get a gigantic contract, and deserving so.