Hi Lisa,
Pilates, yoga, tai chi, fitness balls are all the rage because they strengthen the core (torso) & are generally gentle on the body. We're learning that core strength is essential to all sorts of fitness activities (cycling to baseball) & helps the inactive avoid or minimize back problems. These trendy activities also help people with muscle balance or symmetry. Obviously all ball sports are assymetrical & foster an inbalance but even running & cycling enthusiasts can suffer from aches or injuries caused by muscle inbalance. We all have a dominate leg, arm, eye along with a dominant hand.
These type of workouts can also help with flexibility which not only helps prevent injuries & relieve aches but improves performance by increasing ones range of motion.
I don't think Kries or Stott earned their physiques through Pilates matwork alone. The probably do plenty of work on the Reformer which looks like a midievel torture device but functions as a horizontal Universal set. You can stand or sit on it. It has pulleys which you can hold with your ankles or arms with resistance provided by springs. The Stott website shows several pieces of Pilates equipment. One could certainly do all of their strength work on the equipment. If I won the Lotto, I'd love to do 2 weekly sessions on the Reformer with a trainer. (And head to Europe with my bike..!)
In mat classes, we used rings, bands, fitness balls & even hand weights for arm & leg resistance work. I didn't consider it a replacement for my strength training because we'd only do 1 set for each muscle group. But it could provide all the strength training one needs. You probably have to decide how strong you want to be to determine if it would be adequate for you. Stott has videos that use the ring & bands for strength work if you're interested. Stott recommends combining Pilates with cardio work for a complete fitness program.
Personally I'm using Pilates mat work for torso strength, flexibility & diaphragm breathing. Still doing my strength work with iron, some traditional ab work & heading outside for most cardio. I love how I feel after a Pilates session - it's nice not to feel wiped out!
Debra