Well, lust, especially during the initial phases of a relationship helps in breaking down a lot of the barriers between the two people in it. It makes them feel more comfortable with each other in their own skin. it also leads them to be open and honest about their inner most core. this inturn can help the longevity of the relationship even after the lust factor dies down.

On a flipside, if not reciprocated it can imbalance a relationship. Having had my share of relationships, I find lust a great ice-breaker. can be a deal-breaker too, in some cases.