Due to Email Sending "safety" reasons, we do require a subject in the email templates so you don't have messages going out without a subject, as prospects tend to disregard any messages without a subject.
Now, if you have your sequence settings turned on to send follow-ups in the same thread, whatever subject line you have entered in a later template in the sequence will be overwritten by the original template's subject line! We have it act this way, as far as the subject line being required for any template, since some email templates may be used in one-off emails or in other sequences where subsequent emails may not be sent in the same thread.