Well, the main thing is that there should be no die-off like you'd get from new rock. Thats the big part of waiting for rock to cycle. All a cycle really is is populating everything with the proper bacteria, then waiting for the population levels to balance themselves. Nice clean rock & substrate thats already doing well in another tank won't have the issues that new rock will. "Uncycled" rock will have a lot of things dying, resulting in the excess waste chems.
Same reason Premium Aquatic's rock, for example, does so well as long as you ship it overnight/2-day. Their rock all sits in big vats and is already cycling. Also, if the rock at your LFS has been sitting for a while, it's already cycling there as well, but then again, some LFS rock can be pretty barren of life...
You don't even need to use wastewater from other tanks. Just rock & some sand for great results.