Your thinking of it wrong, fast travel is unlocked after you explore a certain area. After you unlock that area you can initiate fast-travel.

Yes I know that, but the whole fun of the game is to explore. If you spend hours trying to get back to the Tower, you get more satisfaction seeing it in the distance after all that time than the "nothing" you get from fast travel. You don't know how many times I decided not to use fast travel in Skyrim and that led me on an adventure, I forgot what I was originally doing because I didn't use fast travel and it led me to so many cool places :)

But that is what the Patrol mode is for, if your doing Campaign then people will want fast travel.

Still, I myself like to explore while I go on my primary quest. By the way, when you play campaign, will you be in a lobby with other players but just not interacting with them, or will you be in your own lobby ?

In the beginning I think so, but when you leave it will be private.

Well what if you don't want to explore. Instead, you want to go to your next campaign mission and nothing else. You wouldn't want to stay on a shrike that entire time you're trying to get there.

Well that's the trick :) You may NOT want to explore, but if you see something very interesting like a cave or an abandoned building, you WILL be compeled to go inside and look around for engrams, even if you see a fellow player getting ambushed and is about to die, if you go save him or revive him when he thinks he's a gonner, you might just make a new friend ;)