There are reasonable points on both sides. However, I find the experience to be more likely to boost rather than shatter Lewis' confidence. In some ways, I'd rather he get sent down now, after tasting success, than have him with the Twins and hit a slump and get sent down. To me, that would be more of a confidence issue.
For everybody who says 'he proved he can hit in the big leagues' there are literally hundreds of examples of rookies who have a hot one, two, three weeks in the majors, and then pitchers start catching up to their strengths and weaknesses. Consistently making hard contact in baseball is about recognition, reflexes, and repetition - and Lewis had a pretty big gap in his opportunity to do that on a regular basis. Some more time in AAA certainly can't hurt.