its nice to have one but the problem with that is there's always the tendency of getting emotionally attached to the person. unless the two of you are ready to step up and be a couple, there might be casualties. there are also people in this set up who are happy and contented with what they have. it depends on ur partner's and ur level of maturity. it also depends on how you think of the person and what you expect from them. i think the key is setting the right expectations and sticking to it. as soon someone starts to fall, it can go downhill faster than you think and next thing you know, ur stuck in the rut.