Is footwork important? Only if winning games is important. Ultimate, maybe more than any sport, has layers of pretty arm and upper-body movement that disguise a game primarily decided by feet. Basketball is right up there.
As a handler, I get lots of different body-types defending me during big games. I'm just as likely to look across the line and see Jit (Revolver's tiny handler-defender extraordinaire) as I am to see some massive athlete that I have no hope of running with. Jam usually throws a defender at me like Big Jim Schoettler; in a race with this guy, of any distance, I would need him to be wearing flippers. The hardest defenders to beat in a small space, of any size, are those that are confident and fluid in their footwork.
If I can find a hole in a defender's footwork, I can use that. Do they turn 180 degrees poorly? I'll know within a cut or two. Some defenders love to go side-to-side from a stopped position...fine, I'll start my sideways cuts from a jog, and watch them struggle. Jit (and especially Mike Jaeger, who I am grateful I only have to play against in practice) is amazing at moving to any direction from any direction...I have to make my best move, my best fake, and get a little lucky or be a little stronger on that particular day. Plus, if I do get the disc, he knows where he wants his feet in relation to mine for the mark, and I can't coerce him to go somewhere else with any kind of fake.
Defensively, Jaime Arambula (you might know him as Idaho) is revolutionizing what I think of defensive training. He is taking small moments out of D moves, and translating those into drills. I've been doing this with teams as well...the Seattle YCC team in 2005 may have had a flaw or two, but those guys could flat out mark straight up. They would force throwers to make 1-2 extra pivots per stall count. If you don't think that adds up, go count out how many pivots you make on an average touch, and then add 2 for a game. Fatigue, timing problems, confusion, loss of calm vision...it adds up. We trained by doing short, focused drills on tiny parts of the mark. Idaho's PLU women have had some of the best person-to-person D footwork I've ever seen for an entire college team in either gender.
You know that karaoke move that everyone does for 20 or so yards for warmups? The reason it makes you better is that it builds the little muscles that help make that move on the field. Not that you would ever run 20 yards in that stance...but you often move 1/2 of a yard in that manner, in the transition from moving sideways to moving forward, say. Those transitions are where you can build or lose margins against an equally fast player. If you aren't training for those, then you better be much faster than everyone you play against.