Is Bible College/Institute necessary?
Absolutely not!
Many blessed preachers have never graced the doors of a bible college or anything like it, to preach is a calling from God, some men have the gift some don't and its not like a certificate can earn that right. I know there are some KJB only colleges and that is commendable, but anything else I believe will only lead to the straying away from God's Word and adoption of alexandrian versions and philosophy. My previous Church had a decent preacher, he could reach out, but he wouldn't embrace any bible as the inerrant Word of God and he said that this was because of what he was taught in bible college.
