In the long run, this does not really matter. Nimmo, who turned 19 in March, was not going to contribute to the 2012 Mets or the 2013 Mets or the 2014 Mets. However, in the short-run this is a conservative assignment. In the previous two years' drafts, 12 of the 16 high school position players drafted in the first round began the subsequent season in full-season baseball. However, this year's fifth overall pick, the Royals' Bubba Starling will also be ticketed for extended spring training.
Trends are a nice half-story in player development, but every assignment is dictated by what's right for the specific player. In this case, the Mets clearly feel that Nimmo will progress better by not breaking camp with Savannah.
As a matter of organizational principle, the Mets made a concerted effort to slow down the system in 2011. Keeping Nimmo in Extended Spring training allows that process to continue as it opens up more at bats for other, older players early.