I remember Diablo 2 where we could teleport without cooldown to travel the map very fast, but now with that high cooldown of Teleport, in what situation would I ever use this skill ?
The Diablo 3 incarnation of teleport is geared towards tactical movement, not rapid transit.
Given how squishy the wizard can be on higher difficulties, I find teleport valuable for:
The point is, stop trying to use it as simple movement, and start thinking about it as a defensive cooldown (especially with runes like Safe Passage and Shatter).
You could use Teleport in various ways in Diablo 3:
Teleporting across the map.
By using a Wormhole rune, you could teleport across the map at least 2 times consecutively.
Running away from mobs when on low Health.
This is a good tactic to evade monsters that are going to kill you, or at least sometimes get out of their way due to their damage spikes. Like boss fights, for example.
Positioning yourself for your team quicker.
Good positioning always works for an effective dps role. Usually on the easier difficulties its not usually used, but if you want to get in on the action faster or execute a strategy quicker, teleport works.
Kiting a boss monster.
Sometimes some boss monsters walk faster than you do. (Not just the ones with "Fast" on them) To kite them or at least draw away the aggro from you and onto your teammates, you could always use teleport to kite and disengage from a fight.
Lots of other uses
Teleport isnt always just Teleport. Runes make teleport into a whole new array of combos that you could use with skills. Try it out, mix and match and see if it works.
Teleport is supposed to be used much like the Demon Hunters vault. It is used to get you out of dangerous and otherwise fatal situations. You can also teleport over certain walls and obstacles to shorten your dungeon path to the next area. I think Blizzard thought the old version of Teleport was a bit overused and exploited so they kept it but balanced it out a bit.
Also, Diablo 3 is built around killing mobs and champion packs for loot. Unlike Diablo 2 where the bosses had the best drops. Thus clearing through a dungeon and not killing anything is realitivley pointless unless you are in a rush to finish a quest. There is no longer that sense of urgency to reach a specific area, taking it slow and being throurough yields better rewards.
It's still useful as a travel skill, but with runes like Fracture, Teleport is incredibly useful in surviving and escaping some of the extremely dangerous situations in the later difficulties - you can teleport while in a Jailer's jail, out of a crowd of minions surrounding you, etc etc. In Inferno particularly, 'stand and fight' ceases to be a viable strategy, and mobility and defense reign.