Don't say that if you level shadow you won't have any healing practice. At least until you get mindflay you will be healing most of the time, and even after you recieve shadowform you can still collect healing gear and be able to main heal any instance below level 60. This depends somewhat on the skill of the tank, however.

The point is, you can get healing experience, but not necessarily relevant healing experience.  Healing as Shadow, even with decent gear, is not the same as healing as Holy.  And, seriously, if you're just going to be spending all your time in instances doing healing, you might as well be specced for it anyway, so you can do a better job.