I prefer Aery over Glacial Augment in nost cases. Also, you have most likely never faced an aggressive Janna. I play very aggressive and taste of blood allows for thay. I also dont get shield until level 4 to allow for more poke and movement speed with W. Pethaps you should try a build before trashing it just because you clearly dont understand it.

You are correct they dont, I misspoke in the guide (and will fix it) what I meant is when you build it you can perma slow them because you have W and then you can hit with Q to also slow them, they may not stack but you can chain them. You can also use the Q to get the initial slow off and catch up to them and then hit them with a W to help the team secure the kill. I dont find this to be a waste of money at all, the utility of being able to slow multiple enemies at once with Q is very beneficial.