The jury is still out on that one. In Low-to-mid level fights, the dread war horse/stone slith combo is actually quite effective, but not as effective as a DWH/nightmare combo or DWH/bake combo.
Where stone sliths really shine is when you're hunting with other tamers, so everything that does physical damage will benifit from the stone slith's reduce physical resistance attack.
The problem comes when you're with a group (such as the one I used to hunt with) who doesn't help each other out at all, since most high end opponents where you'll be hunting with groups will require good cross healing to make sure your weak slith doesn't die.
So they definitely have advantages and disadvantages. Right now it seems the disadvantages outweighs the advantages.