Of course, it's useful for levelling too, by making mobs take longer to get to you after being snared and by making them harder to get away from you if they flee.

But you only have so many points. The main thing that levelling specs tend to do is build survivability. That point that you could put in Improved Blizzard is probably somewhere in a survivability trait, or they just didn't think it was useful.

That all being said, if you think you'll get good use out of the additional chill effect, then go for it. Maybe only put one point in it for now and see how it goes. You can always respec (even if it costs). Depending on where they put that one point, you might be sacrificing some DPS or some survivability, but if Improved Blizzard suits your game play, then go for it. Personally, when I was leveling a frost mage, I think I did use it because I liked using blizzard and sometimes leveled in battlegrounds as well.

EDIT: I think I'd confused blizzard with something else, although it doesn't change what I said about whether or not to put points there. I'm just not sure it really is most useful for PVP now most movement-impairing abilities are really "intended" for pvp use.