Yes, it isn't something that should normally happen, but if it does happen, the bag will cause a problem at the worst possible time. It just seems foolish and ill-advised to toss out that extra margin of error just because you shouldn't need it.
You don't always know when you're going to enter a decreasing radius turn or where something will pop up around a bend. Ideally, you'd want to be going slow enough to allow for the reaction/adjustment time, but being humans, we don't always properly judge things.
Around here we have a lot of roads where there's limited visibility and you don't always have the luxury of just assuming the car behind you is going to properly slow. Especially if that turn you need to make is just the other side of a bend in the road.
Just my two cents here, but it's a serious consideration to make when buying luggage. It's hardly unheard of for luggage to kill riders.
When shit does happen, any
bag will cause problem at the worst possible time, a tail bag can cause oscilation and handlebar shake even while moving in a straight line.
Unfortunately riding is dangerous and luggage makes it even more dangerous but it's foolish to blame luggage for everything.
So are tank bags inherently dangerous? No, riders have been using them for years, eponymous luggage companies are making them, and there are many different types and mounting systems to choose from.
Are tank bags more dangerous than other luggage items? Not if they are chosen and used correctly.
Are they more practical than other luggage items? Yes, you can stick a paper map or tablet/gps in their transparent top pocket and use it effectively, you can pay toll fares or present your papers/passport without even taking your gloves off and they are fast and simple to attach and remove.
In the end, our friend asked for tank bag recommendations and I gave him one
If I had even the slightest doubt concerning safety after tens of thousands of kilometers of use, I wouldn't have made the recommendation in the first place.