Well I disagree with you about this. In this movie he was bannished by White Council and movie represented this in some unique way. But if you take a look to Arda History you will find that Sauron was the most cunning being, and many times he fled from danger to safety:
- After his defeat by Lúthien, Sauron played little part in the events of the First Age. (possibly hiding from Morgoth's wrath)
- Armies of Númenór's King Tar-Minastir were finally able to defeat him at a last battle near Gwathló or the Greyflood in SA 1700. Defeated but not vanquished, Sauron retreated back to Mordor and began recouping his strength over the many centuries.
- Suspecting that Sauron had returned and was guiding the Nazgûl, the wizard Gandalf infiltrated the fortress in TA 2063 to confirm his theory, but Sauron fled into the East to conceal his identity.
- Gandalf the Grey made a second intrusion into Dol Guldur in TA 2850, and finally discovered that the Necromancer was indeed Sauron. Eventually, the White Council put forth their might and drove Sauron from Dol Guldur permanently in TA 2941.
Anyway my point is when he is in any sort of danger he flee. ;) So, if in game units simply destroy his Body (His spirit always survive, which is used in this version of game) and he starts recovery it is more reasonable to do that on some safe place, not in middle of battlefield. (During History , after fall of Numenor, he hide (Spirit form, we talk about it in this thread.) and start recovering his body and his power. )