Oh dear. The things some people will do in order to not be alone... ;) There is a common affliction among some people which results in them feeling invalidated and inadequate if they are not in a relationship... Insecurity can lead us to terrible wastes of ourselves, and others. In this instance, this kind of person might forge a relationship with someone who they feel is 'better than nothing'. The filler, until someone better comes along.
Worse than a rebound situation, this kind of relationship is out and out using, and is very dangerous. The relationship can seem completely genuine until one day suddenly it's over, out of the blue, with no warning. This kind of person doesn't feel very deeply, has no view for the future together, and is acting 'available' and on the look out for the next "one'. This can be convenient for someone who is looking for the same shallow connection, or who is fine with a casual relationship that could end at any moment.
But if this person is falling for the insecure one, it is their responsibility to let them know the relationship cannot go further. It is disrespectful in any case to consider anyone a "better than nothing" option. Every person deserves to be valued for who they are, and given authentic attention and care. The mature and responsible thing to do would be to at least let the person know you are feeling vulnerable or unsure, and need some time to see what you want. Then they have to choice to stay and wait, or move on.
These kinds of people need to spend some time with themselves (and perhaps a therapist), to truly find out why they need to constantly feel 'loved' or 'wanted' in order to feel like they are 'enough'. Usually they are lacking an internal self-love, self-respect and self-like, resulting in their need for outside validation. Beware of these people. Get to know their friends and family if possible, and find out what they are like in relationships before you become too involved with anyone emotionally.