It seems to me that reinvesting dividends would just add another chunk with a new investment date, but you seem to be saying that it "resets the clock" on an investment chunk that you already have.
Automatic reinvesting is the easiest option, because I like to just invest and ignore. Also I would rather have my money in the fund and gaining value (hopefully) immediately. Can you explain in more detail what the pros and cons are of doing the auto dividend reinvesting?