I rarely see Ghost used nowadays. I rarely use it myself. Ghost is too hard to be useful late game if you are not split pushing the entire game, and even if you are sometimes Flash is better.
Ghost is too easy to counterplay in crowds/teamfights, specially against low mobility champions that need to stick close to targets like Vlad, Morde, Udyr... You just throw some cc at them before they reach the backline, and you kite them around until the Ghost goes off, if they don't die getting poked/kited before.
These champions I mentioned have a nice synergy between their kits and Ghost, but sometimes it is so easy to shut them down in teamfights if they don't have Flash that it is simply not worth it to get Ghost. They can't engage, or follow up engages fast enough. It really sucks when you don't have any means of instantly getting into range to do your work and you need to walk through all sorts of cc's. Ghost doesn't help at all with that, the time you spend cc'ed is time enough for your target to create a distance that will be really hard for you and your Ghost to cover up.
Now, why give a slow resistance to Ghost? Of course any kind of movement impairment is hell for somebody trying to get in using Ghost, but slows are a special case simply because they are much more frequent than the others. Just think about how many champions have slows, how many slows are AoE, and how many (most of them) of these slows are much stronger than the bonus Ghost gives. I look up the wiki just to have an idea and the list of abilities with slows is huge and scary. I didn't do any statistics, but by glancing I would guess that in average a slow reduces mov. speed by 40% and lasts for 2 seconds. Slows are present in probably every LoL match you play, even when you play a champion with no slow at all, you can still get items which provide you some sort of slow, or you can get Red Buff.
So not only these champions need to go through stuns, snares and the likes, but they will also have to go through a **huge** amount of slows. Well, I remember playing games with Vlad where it was totally impossible to get a good positioning in a teamfight simply because I didn't have flash and was constantly getting slowed while the enemy kited back.
I remember when Riot announced in the pre-season changes that Ghost would be reworked to give a burst of movement speed in the first second, but weaker after it. I don't think this would work at all since, at least in my point of view, the intent of using ghost is to have a more sustained form of mobility, in contrast to flash where you get all the mobility instantly. Making ghost more "bursty" will only lean it towards how flash is used, and if you want the flash kind of mobility, you will probably just pick flash. Instead what needs to be done is to reduce the amount of counter play Ghost has, in other words make it stronger against cc. Obviously we don't want everybody buying mercs and running around like an ulted Olaf with 27% bonus mov. speed so I don't know if Tenacity would be a good idea, it would just turn things around and make Ghost have no counterplay when used by tanks. But since slows are so present in this game maybe it could work giving a slow resistance to it, be it duration or effectiveness reduction.