If you're doing everything from the shoulder, elbow, and wrist with a rapier, you're missing about 85% of its utility. Being 6'4", I can start out refused, with my tip pointing towards my opponent yet more than 8' away from him, and with one foot and a full-body lunge - yes, including shoulder and wrist - cover that distance in one movement. Sure, it's like watching a moon shot come in - takes all day - but people who think they're well outside my range at 5-6' away find themselves in for a big surprise.
Fencing - like almost every martial art - finds its first and finest expression in movement. Yes, there's lots you can do - and very subtle things indeed!! - with only wrist and shoulder, and not just in fencing. But if you focus on that to the detriment of core- and footwork, then a maneuver-capable individual, even if they're less competent with wrist and shoulder, will force you into disadvantage after disadvantage ...