having stronger legs can better you other lifts.
Probably the best way of gauging progression is to make sure you are progressing on the three 'big lifts' i.e. SQUATS, deadlifts and benches. If you don't do legs how can you gauge properly?
Having bigger legs make you look better. Like femimuscles said, a lot of girls find big fuggin' quads the best body part on a man.
Curling, you want good calves huh? Squats and leg press do calves quie nicely so when i come to do more direct work on them, all i need is my 1-2 sets and then i can go home, eat, rest and grow.
If you squat heavy then you will realise that the day after, your whole body is sore! Squats = max growth.
Also, the first question a person would ask you if they know you train is 'how much do you bench?' This is a shame because a bench in comparisson to a squat is inferior. Now I'm not saying that squats are best for chest directly, but they will indirectly promote growth, of every muscle and not just pectorals.
Veni-vidi-vici: you look excellent in that picture. Well done for a lot of hard work!
Curling, look at his picture and tell me how much better he would look if he never did his legs?
Exactly...
And the _rat has the right idea!...
So tell me Curling? Are you still gonna do your legs?