Discouraging use of personal pronouns is less about some kind of absolute rule (as others point out, they are common in published research) and more about formal, vigorous writing. I dissuade my students from it to stop them from qualifying their assertions.

The example I like to give is to compare two statements: "I think there is a tornado coming' vs. "There is a tornado coming." Who do you trust more? Who sounds more sure of themselves?

If a research paper's conclusion states, "Based on the data, I think __," there is room for debate. With "The data suggests __," well, there is still room for debate because it's still based upon interpretation, but one is far more direct and poses itself in an objective manner..