Take a counter-way of thinking. If God is ever-perfect what would force God to take form every now and then? What is it that God cannot achieve without taking form? If there is indeed such a situation where God cannot do without taking form, that makes God imperfect before that. God is all-powerful and all-capable – this does not mean that God can change himself as per his whims or plans, it means that He does not require any external support or modifications to fulfill his tasks. God never has a choice – if God is one who has option to CHOOSE, the very definition or essence of being God fails. God has no option to choose; He is ever perfect at all situations and performs the most optimized management of world at all times.
That is why Vedas very clearly define God as changeless, formless, ever-perfect and all capable at all times without requiring any further changes on temporary or permanent basis.
Another point worth analysis is that all these alternate theories like advait (God and soul and nature are all one and same), and other variants of dvaits (God is with form etc), came in vogue only in last few hundred years or so – in a period when already we were suffering from rule of foreign invaders or post-Vedic philosophies. We need to go back to original source and take an unbiased look to discover the truth.