Sustain was a safety net. The complaints started when everyone realized Titan Egi was suddenly useless as he can't actually eat damage anymore with the heals aiding him, just like a normal tank. Egi's are usually caught in regular AoE heals, so Sustain was super situational in group environments, but since this removes the power to care of a pet from the pet master, it can be terribly frustrating. You really didn't need Sustain that often, but you still wanted it.

Since death of the pet is the second worse DPS loss to a death of the player, this is why it's a critical concern to all SMN. Why would a pet job lack the means to care for the pet?