All hail the MC.
Celerity is fun for aggressive skulks, rushes, and counterrushes.
Adren awesome for bitespammers, sporesuppression lerks, or healspammers.
Silence great for any ambush players.
Silence gets special mention because it really throws people off - most players are listening for the enemy more than anything else. NS is a pretty dark game so most players with average gamma is going to be rather shocked to be on the receiving end of bitespam from nowhere.
Second, MC means hive teleports, which is very VERY good for hive defence, and hive defence is certainly important when most marine players will blindly go for hive lockdowns. For an aggressive earlygame, MC wins. The problem is if you switch to the defensive, you'll get slaughtered.