Whenever you might want to use B&S you can use a feather instead. Then if you happen to wish that B&S lasted longer, you can just drop another feather instead.
That's been my thought since the begining, yet I still see people preferring B&S over it. I don't even get it.
On top of that, they don't ever disappear, so you can set them up, and forget about them. Kinda like the Showtime Rotisserie.
Set them up, and if you don't use them for 20 seconds, you can hit 6 consecutively, all 3 you set up and then 3 more as you are running.
There are reasons to consider body and soul. First, Feathers go to the first (friendly) person to touch them, so you might not be able to get them yourself. Even if you cast them in your path if someone is running with you they might take first. Second, B&S heals while it gives you the speed boost, so you don't spend a global not healing. Lastly, you can use B&S on more targets in a short window even if you can only use it on each one once (twice with Lifegrip added).
There are better reasons to choose feathers though. You can use consecutive feathers on yourself or a single kiter for more and longer speed. Feathers don't cost any mana, so if you don't specifically need a shield on whoever you want to speed boost, you don't waste any. Feathers are never locked out "by default" by regular healing, so you will always be able to speed boost when you need it vs running into weakened soul at an inopportune time. Feathers also work well with multiple priests. Also, if someone grabs your feather, you can just cast another to grab yourself most of the time, so that shouldn't be a big enough issue to make you chose B&S.
Doesn't work very well when you're stacked with numerous other people- especially in a 25m. Additionally, for the same gcd, you also get a shield.
Consider how that situation works out. Either everyone needs to move at once in a stack, or specific people need to move out. If everyone needs to move, feathers aren't a good choice, but neither is B&S (huge cost, staggered speed boosts across the raid). If only one person needs to move, instead of casting it on the stack, you can cast your feather just out of range so you grab it as you move out. If you don't need to move far enough for that to work out than the speed boost isn't going to do much good anyway. In fact, if multiple specific people need to run out, you can surround the stack with feathers, and anyone who runs automatically gets the speed boost when they need it.