I guess it depends on what your goals are, you will certainly build strength through yoga moves but if you are wanting to actually build up your muscles I think you really need to do weights.
Right now I am wanting to lose weight and tone up all round as well as maintain flexibility so I alternate both yoga and light weight circuits with cardio. Down the line I do intend to increase the weight workouts.
Also I don't know if you've ever tried the pilates reformer but for me and my body I got the best results using it. I have long limbs so for some reason these exercises work best for me, particularly for sculpting arms & Shoulders. It can be expensive but welll worth it IMO.