So we’ve got a well-rounded view of the kettlebell swing. It’s mainly about the hip hinge, and the energy created by coming out of that hinge nice and briskly is what gets that bell swinging.
I think the optimal swing is one with a minimum of knee flex and a maximum of hip hinge. This means that with a heavier bell, your back might be nearly parallel to the ground at the bell’s lowest point back between your legs. This shape puts the most emphasis on your posterior chain – hamstrings, glutes, low back. This is a good thing.
Now then, it’s also possible to execute a good swing with MORE flex at the knee, and therefore less flex at the hip. This will bring the quads into play. This is also a good thing if you want to work your quads more. Maybe you find squatting uncomfortable or difficult, for example, but still need to develop quad strength for, say, cycling or hill running. And you’re not keen on dong a lot of cycling or hill running. Maybe you find the maximal hip hinge a little uncomfortable, and a little extra flex at the knee makes the effort less concentrated on the posterior chain.
So if you’d rather bend your knees a bit more, be my guest. Just be aware of the pluses and minuses of both styles.