It wouldn't hurt at all. If it is not unusual for petitioners to file more than one petition at a time (e.g. NIW + EB1-EA) together with a concurrent I-485, then there should not be a problem if more than one gets approved. As to why people withdraw their second petitions, it could be a number of factors:
1- They have concern that, with one approved petition, they want to base their upcoming I-485 on that and not get things muddled with other petitions, especially if the other petition is a lower category which might confuse the adjudicator because the lower PD is not yet current.
2- They get an RFE for the second petition, and either don't respond or respond by requesting withdrawal instead of going through the trouble of responding.
3- They just do it "to be on the safe side" for whatever reason.