I know its easy to fall into that belief, but it's not true. There's a reason so many champions default to wearing
over defensive boots or offensive boots:
You can't use your kit enough per game if you are not mobile.
For example, a 356 Move speed
versus a 498 move speed 
There's no competition between the two. A slow Zyra cannot keep up with her team, gets far less participation/shutdowns, and is basically only good for defending and clearing lanes. Unless she's occasionally setting up a pre-eminent bush gank where she knows someone is going to pass, she generally is defaulted to being dead weight.
A Relentless Hunter Zyra can both outrun her chasers long enough to E them successfully and prevent a successful chase, possibly killing them with very low health, and be in almost every fight.
I really can't tell you how many times I pick Zyra particularly versus
. Voli's speed boost can almost catch a 498 Move speed Zyra. He would catch and kill a 356 move speed Zyra.
This is why I generally argue that Phase Rush builds are generally heavily underrated as well. Having the mobility to move around on immobile champs completely changes how they work.
For example, I can outdamage an entire team on pure movement speed runes on
...... simply because I become omnipresent. A slow moving Karma is generally an easy target, dies a lot, and becomes a Q bot. On movement speed, I hit about 30-40 more skills a game! Each hit might not have a Rune's damage, but the sum of the damage does FAR more to proc Karma's passive and get out more innate kit damage to compensate.