You also don't get hard-counterd by aoe comps as Zyra. Sona pretty much has to stay really close to someone for her auras to do anything.
I've seen this sentiment a lot and I'm fairly sure it's the opposite of the real situation - it turns out that bringing a free, high tuned Locket (and you can still buy another locket!) is really good against AOE comps. It's not like it was hard to catch multiple people in an AOE before - you don't need a Sona on the enemy team for that to happen, that happens pretty much always, at every elo, anyway - but Sona blunts the effects a lot.