You're right in that defense has traditionally been seen as less valuable by both scouts and statisticians, and that lag causes defensive-minded players to perhaps become undervalued.

The issue is that Iglesias' offensive numbers were so awful last year that it's looking like Escobar's offensive numbers last year might be closer to Iglesias' ceiling rather than a reasonable projection. And when your offensive ceiling is a sub-.300 OBP and a sub-.650 OPS, it's really, really hard to be taken seriously as a prospect. If Iglesias' offense does not progress, is he all that different of a player from Che-Hsuan Lin? Granted, the offensive bar at SS is significantly lower than it is at CF, but they both fit the profile of elite defensive players with a very limited offensive ceiling, and that sort of player relies on less-certain defensive metrics to define their value. For better or worse, UZR or Totalzone or whatever other metric you use will still be significantly less credible than the standard offensive stats, and it makes some sense to be hesitant to embrace players whose defensive contributions make up the majority of their WAR.