In the PvP meta right now, you can only take either a horse or a thrall with you. The best counter to a horsemen is another lancer, or a fortified and elevated position. If you are allowed to take both a thrall and a horse; it is no longer a tactical choice. Imagine a six vs six scenario; before this change, some players might choose to take thralls, and some might choose to take horses. If you had a thrall and not a horse and you were outnumbered by horsemen, you could choose to take an elevated position and play with your thrall to protect you from enemy cavalry. If the cavalry decided to dismount and climb after you, they were risking their lives and horses to an enemy thrall. In this scenario, there was a chance for counter play.
The horsemen can either disengage or take a big risk. Now, they don’t have to do either. They can just get their own thralls to mop up your single thrall and then take you out. In our six vs six scenario where everyone has horses and thralls; the outcome of the victory hinges on which team is dismounted first, running away from a horse is hard enough but when you are knocked off your horse you will be surrounded by enemy thralls as well; your horse will probably die and you will be ran down by cavalry.
SUGGESTION: Buff Pets a little bit. Make it so that player can have a follower and a pet (including horse) that way players have a choice between taking two ground fighters or a thrall and a horse. Or maybe just let us have two followers, period. Any two followers. Then you have the choice of taking two thralls or a horse and a thrall