E is not a powerful movement speed buff until late game after you've leveled your ult and have some AP. Early game (pre 6), the ms buff is very small, and the ability is pretty much only useful for it's powerchord.
SELF BONUS MOVEMENT SPEED:** 13** / 14 / 15 / 16 / 17% (+ 7.5% per 100 AP) (+ 2% x Crescendo.png Crescendo's rank)
That means that just considering base ms at level one (325), if you used E you would get a speed boost of 42 to equal 367ms that decays to 357ms over 3 seconds.
That means that you will basically be walking a little faster than a bootless Lee Sin for 3 seconds. The ability is absolutely *not * spammable early, seeing as it's got a 12 second cd.
Now, consider the other supports that have a base 325 movement speed - Soraka, Zyra, Lulu.
All of these champions have a way to peel for themselves if they get jumped on early (pre 6). Soraka has equinox, Lulu has polymorph, and zyra has grasping roots.
Sona literally has no form of cc to protect herself or peel, outside of E's powerchord and her ult.
Meanwhile Nami has 335 base ms, she can use bubble to peel for herself/her carry and she get's speed boosts just by her passive. Her ms was a crazy 340 up until patch 4.13, which made her have one of the highest base ms of any support.
**This nerf just doesn't make any sense in regards to Sona's kit and how she aligns with the other supports.
**