He really didn't do a bad job at 1B. He was adequate. He never hurt us too badly in the OF either. He was slow with a weak arm, but he postioned well and he had good instincts. He looked awkward, but the results weren't bad.
As for the other OFers you mentioned, we had Roberts for only a couple of months. He rarely played. Nixon was on the DL for a good portion of Millar's time in Boston, so Millar played a lot of RF. Also, once Ortiz had proved that he could handle the DH spot, there was no reason to move him into the field. Most guys hate DHing ad don't do well as everyday DH's.