Another interpretation: "For it is good for him with you" (Deuteronomy 15:16) - if he was lame or blind, he is not pierced.
A Lame or Blind Servant Cannot Choose Permanent Service
Yalkut Shimoni on Torah 315:2
דָּבָר אַחֵר, "כִּי טוֹב לוֹ עִמָּךְ", אִם הָיָה חִגֵּר אוֹ סוּמָא, אֵינוֹ נִרְצָע.