There is one underlying reason why travel times continue to be prevalent in MMOs today and it's directly related to player expectations of virtual worlds. Developers are very sensitive to the issue of making their games seem like a real world and not a collection of three dimensional video game levels. Any MMO that has too many instances and separate zones often gets complaints for breaking the feel of a seamless world. This can actually turn players away from the game. As a result, the most successful games are often those with an over-world where players don't find themselves constantly having to load data as they zone. Just look at how World of Warcraft, Lord of the Rings Online, and EVE Online handle their world design.
Developers can take this overboard though and make worlds that are a little too massive in size sometimes. Games that don't properly fill in their world with quests and mobs have a boring and monotonous feeling to them. However, even when a massive world has been properly filled with content, it can still be a pain to travel through. Players often have to meet up with their friends in specific locations and travel time can become a limiting factor against forming a group. Mounts have become a common solution in MMOs that allow players to decrease their travel time without reducing the size of the actual world.
This isn't the only reason why we continue to see mounts being included in most MMO designs. Some wise guy working for Blizzard quickly figured out that mounts make a good status symbol and World of Warcraft soon had a wide variety of them. Players just cannot get enough of mounts offered through those limited time special events and holidays. This demand for a unique mount continues to exist even after the events are over. This has made them into the perfect money sink for games. The more unique and fast the mount, the more money it can remove from the economy.
Mounts aren't just a simple travel mechanism anymore. A lot of games have integrated them more directly into their game play. Age of Conan has introduced mounts with different abilities, which become useful in different types of combat situations. Giant war mammoths are slow and almost useless for fighting a person, but they do great damage against city walls. Horses on the other hand allow for quickly running down any opponents who happen to be on foot. The mounted combat in Age of Conan still has some kinks that need to be worked out, but other games have also integrated mounts and vehicles into their combat.
EVE Online uses no avatars at all and its players experience the game only through the cockpit of different space ships. You could say one is always "mounted" when playing EVE Online. This doesn't seem to detract anything from the combat since the game often receives praise for having exciting and strategy filled battles. It's not for everyone though, since a majority of MMO players are used to being directly involved in combat though an avatar. In fact, EVE Online is the only successful game I can think of where players spend a majority of their time in a vehicle.
Auto Assault and Pirates of the Burning Sea are the other two widely known vehicle based games, and they both have a checkered past. Auto Assault did not include any sort of avatar interactions and had problems capturing enough subscribers to keep itself afloat. Pirates of the Burning Sea on the other hand rushed out poorly designed avatar combat at the last minute. Many reviewers criticized this decision and claimed it was only included to increase market appeal. This might be the case since the history of vehicle based MMOs in the Western market is pretty bleak. Eastern games have had more success with hits like Exteel and Kart Rider becoming popular.
The Western market does not seem to have the same fascination with vehicle based combat unless the MMO involves space ships for some reason. As a result, the majority of MMOs are following what the market seems to want and staying away from games that are strictly based on mount or vehicle based combat. That's not to say mounts and vehicles are going to remain just for traveling. Mounted combat seems to be in demand and even more exciting is that several games are experimenting with aerial combat. Footage from some of the Wrath of the Lich King previews highlighted players fighting while mounted on Wyverns and Gyrocopters. Other games are sure to follow in the footsteps of World of Warcraft if this catches on.
More evolutionary than Blizzard's aerial combat are the different space based MMOs currently under development. Jumpgate Evolution is primarily a vehicle based game like EVE Online, but seems to be following a more casual friendly philosophy. On the other end of the spectrum, we have Blackstar, which uses an impressive mixture of avatar and vehicle based combat. Both games are setting an example by using vehicles heavily throughout their game play. What this means for the future of mounted combat in MMOs is hard to say. Chances are strong though that we're seeing a trend of moving away from treating mounts and vehicles as simple modes of transportation. While they still fulfill our need for speed, they also have many more applications in MMO design.