Think of it this way, ignoring the fact that Sona is indeed less likely to die when clearing wards
If the support takes yellow trinket then you have no sightstone. If you have no sightstone then your overall vision control options are down from 6 items to 5 items, since no one else can effectively afford to buy a sighstone except maybe your jungler. If you take a sightstone you should take a sweeper, since the yellow trinket is redundant at that point.
If your support takes sightstone and sweeper then your AD doesn't need sweeper; its redundant because your AD and support will likely be working in tandem. The blue trinket, sightstone, and sweeper, can all combine so that the support and AD quickly and efficiently clear wards while having overall more utility.
Moreover, someone on the team needs to take a blue trinket. That person should be one who roams the least and makes plays with the team for objectives, since people who are roaming more need to sweep or place advance wards. The champion who roams the least and tends to be behind other champions the most is the AD, so they should be taking the blue trinket.
The overall vision issue becomes even more apparent as you add blue and yellow trinkets to the mix since it lowers your overall sweep capacity. Which lends more credence to the idea that supports should buy sightstone. And of course if supports are buying sightsone then they should also buy sweeper