In a word, no.
Here are Guzman's career stats:
These aren't the stats of a consistently productive player. In fact, this season isn't an outlier--Guzman has actually had a season in which he hit even worse than he has this year. His 1999 stats were frighteningly bad. The only adequate year he had was in 2001, in which his OPS was .814. If we take away that year, which he has never duplicated, and his two worst years, we get remarkably consistent performance (when we say "consistent performance," you can read that to mean "consistently bad performance"). If that's what we can expect, then Guzman will have an OPS of about .685, and we'll pay $4.2 million for it.
How does that compare to the performance of other shortstops?
In other words, even if Guzman returns to his "historical" performance, the Nationals could get the same or better performance for a fraction of the price. There simply is no argument that Guzman is worth $4.2 million, even if this year isn't representative of what he'll do in the future. That's why Minnesota was shocked when The Exciteable Boy offered Guzman that humungous contract.