I most often run into people dating while separated when they're separated themselves and involved with someone else who's separated too.
A more accurate term for 'separated' in most of these cases would really be 'separating,' since few of these people are actually through their divorces or have completely ended their previous relationships.
Dating while going through a divorce can have a number of negative effects on the divorce proceedings, both in court and emotionally.
Additionally, while every state is now a no-fault divorce state, marital misconduct can still be considered in some situations.
And with the prevalence of cheating, a lot of times one partner has already begun a new relationship and the other partner is now seeking to get 'mine.' Once it's been decided by one, or both, partners to end the relationship, most typically both of partners start seeking a new relationship. If you really care about your new love interest then you'll apply more reason than emotion to your decision about dating while separated.