When is it more beneficial to have Ghost over Flash?
I always thought that the Ghost Summoner Spell was essentially the more "noob" version of Flash. Recently, however, I have seen several pro-level games (MLG, PAX, etc.) where the pros do indeed get Ghost instead of Flash. I don't understand the logic behind this and was hoping someone could enlighten me. I've seen it a couple times, but the champion I specifically remember it on was a Shen.
The only thing I could think of was that Shen can already use Taunt to get over walls and obstacles, so perhaps Ghost would generate more distance in the long run... But even this seems like a stretch.
Thus the question: When and why is it more beneficial to get Ghost over Flash?
The way I think about it is like this:
if your champion has a snare/root/disable/whatever, then Flashing in range and using it (pretty much) guarantees you can keep hitting them for a good while at least. Think Darius and his W-E combo, you can Flash in range of a champion, pull him to you and snare, then just keep hitting him.
if however your champion has no such disable, Flashing alone will get you one hit, maybe an autoattack too, then they run away. Here Ghost is more beneficial because you can keep chasing them. Shyvana is a very good example of this, although I like Ghost on Fiora also because her sprint ability is kind of short duration compared to the CD and she has nothing else.
You should however consider that Exhaust might be a better choice over Ghost for chasing/ganking purposes where Flash isn't "the right choice", because in addition to the snare, it also nerfs their damage output, so you can use it as a peel for your carries (or just straight dueling). I use Exhaust on both Nocturne and Shyvana for this purpose.
The argument above that Ghost is good at escaping... I don't know about that. You never really leave melee range if they snare you during it, whereas Flash opens a gap for you to let your snares run out and stops damage income cold. Ghost is also a one trick pony, whereas Flash can net you a huge tactical advantage by Flashing over walls that champions without Flash would have to otherwise slowly run around, or lets you escape turret damage instantly etc etc.
Try them all in my opinion, you'll get the hang of their strengths and weaknesses after some amount of games. I'll tell you that I miss Flash the most when I run Ghost or (more often) Exhaust, but I'd still never run a Flash Shyvana.
There are only very few Champions where Ghost has an advantage above Flash, mostly champions that have an ability that is very similar or champions that have abilities that prevent them from getting stunned or snared.
The champion that is most often used with Ghost in highest level play is Olaf. His high base movement speed and slowing axe, as well as his Ultimate of course which prevents any crowd control effects, make it easy to escape dangerous situations so Ghost allows Olaf to chase down opponents more easily.
Other champions that are sometimes used with Ghost are Master Yi and Tryndamere which both have abilities similar to Olafs Ragnarok.
In general, Ghost has the main advantage of giving you a lot more chasing power which is especially useful for melee bruiser champions. Flash gives you an escape with the advantage that you can use it across walls, making escaping and positioning a lot easier for squishier champions like AD or AP carries.
Before you now run off and use Ghost on every Bruiser, note that Ghost does not prevent crowd control effects so if the enemy has any slow, you still won't be able to reach the enemy carry and you won't even have the option of escaping that you would have with Flash.
The only champions i would build with Ghost are the above mentioned Olaf, Yi and Tryndamere. All others should always have Flash.