Actually it's been shown that through visualization it's possible to enhance the cortical output signal, which increases strength by driving the muscles to a higher activation level.

It doesn't have the same effect as workout, but it will have a (possibly strong) effect.

A lot of people in the body building world attest that if you visualize working out while you are doing it, that you build muscle mass and control much quicker.

How strong the effect would be is unknown, but there is a very large likelihood that workout in dreams does have a positive effect on your body.

It makes logical sense, a lot of things cause a similar response in the brain, whether doing them or visualizing doing them. Large chance that extensively dreaming about working out will cause your brain to release a growth hormone of some sort.