Others have pointed out that travel powers aren't often used in combat, in fact they are suppressed while engaging in combat *(briefly, while not PvPing). I think that's the right way to look at the travel specific IO sets. Do they improve the ability and also add bonuses that are unique to each IO set travel power respectively? Yes, I believe so.
As the point has developed in this thread, why dedicate slots to a travel power? Why balance the potential reward of 4-slotting a travel power versus 4 slotting another power that can provide better bonuses catered to the users build objectives? The answer is in the question - players are given the option, the flexibility, the mechanisms to make their character how they want them to. Some players don't care that their 4 slots could be utilized in a different power to add more bonuses - they just want their travel power to be the best it can be. And that can be achieved by slotting each travel IO set to the corresponding travel power.