Think of it this way, how often do you pick things up with your arms in a curling or similar motion? How about your legs? I think the fact that the legs are fixed/grounded in most force directing movement and the hands are free mostly explains this.
I am not saying that leg curls are useless, just that the benefits of doing a motion closer to how your body actually moves and directs force is likely to have a better effect. Leg curls generally lack the contraction of the glutes and spinal erector muscles to the degree that they occur in life or in other exercises that target the hip thrusting movements.
Certainly in cases such as bodybuilding of rehab/injury prevention an argument can be made for their use, but with more benefits from movement patterns like deadlift and squat variations (including single leg variants), it is harder to justify their use on a frequent or voluminous basis.
As for biceps, they also respond quite well to chin ups and dumbbell rowing, so an argument can be made against their over-popularity, but alas is lost on the bench and biceps crowd.
If it works well for you, do it, who cares what some chucklehead thinks. You know what they say about opinions and assholes.