You are right, of course, about the value of spontaneity in the course of personal prayer. It's like spontaneity in love talk with your spouse. There's lots of opportunity --- lots, and it's simply indispensible.
There's a time for everything: a time for private prayer, and also a time for liturgical prayer. The difference is like the difference between love-talk and marriage vows. In marriage, you're not just expressing impulsive heartfelt twosiness for right now: you're pledging in public to the permanent institution of marriage, which is bigger than just the two at the moment. It spans lifetimes, it brings new life into being, it creates durable, recognized kinship, it links generations.
Liturgy is like that. It doesn't take the place of personal prayer; it's the public kind, it links the whole Church, "from the rising to the setting of the sun," past and present, east to west, with angels and archangels and saints and martyrs, in heaven and on earth.
It's therein that the priest was going off track. Does he want the opportunity to be creative and innovative, to do his own personal thing? Great. He can do his personal thing 23.25 hours a day. But during the Mass--- he should forget himself. He is in persona Christi. He is a priest with Christ. He is praying with, on behalf of, and within, and for, and as, the Church.
Peace be with you!
Great response, Mrs. Don-o.