It depends on what you mean by 'strength'. KBs etc. are not the best tool for building maximal strength. Barbells are. That's hard to argue. If you're looking for power output endurance and combining aerobic and anaerobic conditioning, KBs are a pretty good tool. I use them to prepare for BJJ tournaments as well as keep weight off (they burn fat great because you just do so much damn work, in the physics sense, in a typical KB workout).
I think for people who haven't done any strength training you'll probably get the most benefit from a basic barbell program with the big lifts (squat, bench, deadlift, and press, cleans if you like), but when you start plateauing doing that few days a week or find that it's taking away from your grappling KBs are excellent for staying strong and building conditioning. In general though those things are conditioning tools, not really 'strength' tools per se.