The action of Metformin is to work against Insulin resistance, thus it is mainly used by T2s. It will help a T1 who has become insulin resistant. This question reads as if some doctors think that Metformin is an insulin promoting medicine like thesulphonylureas, which are useless to all T1s. If a doctor wants to change your meds, especially if you've been getting on well with them, ALWAYS ask why. I would NEVER take a medication if I didn't know what it does in the human body.
I'm a T2 and find Metformin is all the medicine I need to keep my numbers on target, that and the reduced carbs. All my medicines have been decided by my doctor AFTER discussion with me. then I am lucky enough to have a great GP ( I've never seen the diabetologist)