I think her movement speed ability, prior the change, was relatively weak and usually the spell most Sona's maxed last.
I don't mind having her zip around in the late game because if she gets touched, she dies or has to use her ultimate to disengage.
Building full AP sona, in order to get very high movement speed, really makes her even more susceptible to getting killed. Any assassin can kill her in one rotation of their abilities and can get strong off her deaths in team fights.
Most players still don't max out the movement speed early, and if it goes to late game where damage is extremely high on most carries, I don't mind her having higher mobility to avoid danger and to ward. Warding with Sona still feels pretty dangerous compared to other supports and having higher mobility significantly helps in that regard.
Prior the changes I mained Sona if I had to support. She was banned so often, I had to pick up other supports (more tank-like ones).