Plucking technique is one of the things that some people get all uptight about. Known techniques (2 finger, 3 finger, thumb, anchored thumb, floating thumb, etc) are all just guidelines to help us know what works well for others. There is nothing "never" or "always" about them. The reason it is recommended to always alternate is to get muscle memory working - sometimes, the fingers will get tied up over a certain line and if your plucking is not automatic, it can be troublesome. But for me, never say never.
It is very widely accepted that the 2 finger method is one of the most efficient methods. But even within that method there is the option to rake (use the same finger) when moving up a string. So even within the confines of the 2 finger "formal" technique there is room for using one finger consecutively.
Bottom line is: Whatever it takes for YOU to get the job done is right. It may be harder than it has to be, but if it works it works.
That said, I learned to play with a one finger approach and have worked hard over the last several years to integrate 2 finger plucking into my playing because it does allow for more speed. I rarely exclusively use 2 fingers - I just let them do what it takes to play the line.
Yes converting to a new plucking method is brutal if you've locked into something else. If you don't use two finger a lot, I'd advise to try to work with your instructor and try to integrate it more. But if you're happy with your playing the way you do it, I wouldn't conform to his "always" demands.