`nvm use system` should work as expected.

nvm.sh

@@ -550,7 +550,17 @@ nvm() {
           VERSION=`nvm_version $NVM_RC_VERSION`
         fi
       else
-        VERSION=`nvm_version $2`
+        if [ $2 = 'system' ]; then
+          if nvm_has_system_node && nvm deactivate; then
+            echo "Now using system version of node: $(node -v 2>/dev/null)."
+            return
+          else
+            echo "System version of node not found." >&2
+            return 127
+          fi
+        else
+          VERSION=`nvm_version $2`
+        fi
       fi
       if [ -z "$VERSION" ]; then
         nvm help

test/fast/Running "nvm use system" should work as expected

@@ -0,0 +1,13 @@
+#!/bin/sh
+
+die () { echo $@ ; exit 1; }
+
+. ../../nvm.sh
+
+nvm_has_system_node() { return 0; }
+[ "$(nvm use system 2>&1 | tail -n1)" = "Now using system version of node: $(node -v)." ] || die "Could not use system version of node"
+
+nvm_has_system_node() { return 1; }
+[ "$(nvm use system 2>&1 | tail -n1)" = "System version of node not found." ] || die "Did not report error, system node not found"
+nvm use system 2>&1 > /dev/null || [ $? -eq 127 ] || die "Did not return error code, system node not found"
+