lol if its a tiny modification that then is almost perfect for party goers. can test whether you are ready to go. it can also help with medical testing(the lil bit it can). but stil it can be of some help.
a body fat sensor with a heart rate monitor would be really great for working out. combine those with GPS to measure distance over time and the accelerometer to count steps and you could have a very nice set of metrics!
I think a Body Fat sensor is a great idea, we have a ton of unhealthy weight loss where people diet, cause their body to lose muscle mass, and balloon right back up. If the difference between Fat content and pure body weight got more attention I think we would all be a bit healthier