Ground vehicles don't even EXIST in Star Trek. They have transporters and shuttlecraft for all that. Yeah, they had a few jeeps in Enterprise and Voyager back when they had to get involved in World War 2 and when Tucker and Archer ended up getting involved with a race of alien humans who had a pre-information age society.
... oh, and Tom Paris' Camerro from the holodeck in "Vis a Vis."
No, we don't need mounts or anything to actually speed up ground movement. It doesn't work with 5 man groups anyway. You can't just expect everyone to just remain seated during combat.
"My frozen dairy-based confectionery attracts all the males of the species to the facilities. They all agree on it's superiority. Indeed, it is superior to yours. I could teach you the finer details but that would require monetary recompense on your part." -The Milkshake Song: Vulcan Edition
Having a ground vehicle in STO makes as much sense as taking your car to go to a store that's a block away. we have tranporters that drop you within walking distance of everywhere you have to go, what the hell would we even need ground transport for?
This falls in the "Let's make this game quick, easy and lame" category, thanks but no thanks.
As others have stated, the main purpose of mounts in MMOs is to speed up travel between regions. Given that Star Trek features starships that can hop between star systems in "reasonable" amounts of time, as well as shuttlecraft and transporters, dedicated ground vehicles aren't neccessary.
The whole Argos sequence in Nemesis was just stupid, why use a dune buggy when you can cover the same ground faster in a shuttlecraft?
Ground maps in the game are simply not big enough to justify a ground vehicle, personnel moving around on Segway like pods would be well exposed targets for enemy combatants.
Jet packs might have a use if the maps were designed for multilevel activities where higher areas are only accessible with said pack, but since none of the current ground maps have this, the device isn't needed.
This isn't a game with huge ground maps like WoW etc., so a mount would be just shaving seconds off of travel times.
The Argo isn't unthinkable, but it was misused in Nemesis. There a shuttlecraft would've done easily enough. A ground vehicle like that might be good for environments where prolonged shuttle flights and transporter operation both are problematic. Charged storms and whatnot. Of course, then a vehicle that leaves the occupants exposed to those same elements is probably not the most sensible choice. Though they may be rare, there's probably circumstances where a ground vehicle would be convenient even in Star Trek. (Just not so common that they wouldn't replicate or build one when the need arose, rather than take up a parking space on the shuttledeck for a scenario they may never encounter.)
That said... there's no place for mounts in STO. Virtually all of the maps are too small to justify them. To add mounts they'd need to add new maps first that are massive enough to make people feel they need that speed boost. Moreover, with all the customization options for STO characters, the team couldn't even get all characters to sit down in the same chairs properly. Some end up sitting with their backs a foot from the backrest, others have their feet (or tail) stick through the bottom. That's a problem that'll reoccur all over again if they try to add mounts.
Rocket boots might work though.
Or personal transporters. Though I suppose both could break missions when someone clever beams/rockets to the high ground and picks off a boss at his leisure.
This all has me thinking though, on the subject of the larger map needed for mounts: I don't think one map would be enough to warrant buying mounts. But there could be a middle ground. A massive 'ground' map, for a shuttle mission. That would be a mount-ish experience, I think, and could be a lot of fun. And help push to sell whatever shuttle(s) they no doubt will come up with along side it.