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You've got to guesstimate
Taylor = 13 WAR in 6 years, x 70% confidence = 9 WAR (maybe)
Marte = 10 WAR, x 45% confidence (that he'll play that much) = 4.5 WAR (maybe)
And then you've got the nebuluous questions:  
1.  What are the imperceptible differences between having a PLUS shortstop vs an AVERAGE shortstop?  Which go far beyond what WAR captures.
2.  But what about all the guys behind Marte?   We're just considering ONE option to Taylor.
3.  What about Brad Miller?!  What about Scenario A in which Miller or somebody plays SS and then you have player Y in right field?  A poker player would draw out Scenarios A-G and average them.
4.  What about the in-season performance cost of "trying out" Trayvon Robinson and then Abraham Almonte?  
5.  What about the pennant cost if, next year, Taylor is a catastrophe?
.........
But one thing's for sure:  once a player hits in the bigs, the confidence goes way up.  And affects the math greatly.  It should.