The pros and cons have already been pretty much outlined for you. They can be neat additions to the cage, and if your skink doesn't poop in them, they don't get too messy. As mentioned, a thorough cleaning every so often is needed to ensure the pump continues to run smoothly, and loose feeder insects can and will drown in them.
I actually really like using waterfalls for baby beardies and baby skinks - the running water encourages them to drink, and baby beardies in particular are kind of dumb when it comes to finding their water source. My skinks never had much of an issue, but since they're shedding more often, the added humidity can be helpful.
Go with what you think looks cool

If you've only got the one skink and you've got the space and the willingness to clean the waterfall occasionally, I don't see the harm.
-Jen