NHL contracts doesn't mean "need development". For example, Kovacevic has a NHL contract but doesn't require development.
Right now, we have space to play everyone, but no rookie could make the roster. Example:
Matheson - Guhle
Xhekaj - Savard
Struble - Barron
Harris, Kovacevic
Hutson - Reinbacher
Engstrom - Mailloux
Trudeau - XXX
The reason why we would need to trade a D is not playing spots per se, it's NHL roster spots. By trading Harris for example, you allow a rookie to push to make the NHL roster.
You also have to consider that there is a decent chance Savard and/or Kovacevic don't finish the year in Montreal.
For the record, I am for trading both Harris and Ylonen (or the dead wood) to make space at the NHL level for rookies otherwise the roster is pretty much already set.