The real question is are you willing to stale bthrow's killing potential for positioning? Personally I only do it if it'll put them offstage. It's certainly not worth staling it just because it could do 2% more than fthrow.
Even if the opponent DIs the balls perfectly, they're still sent up in the air really high and far away, even the fastest falling characters need some time to get back down. You're not necessarily trying to capitalize on their descent, just using the time to get a breather and plan out your next move.
How does dthrow kill at the percents you mentioned, you sure you're not mixing it with uthrow? And it doesn't really combo at low percents either, just that the window for escaping is smaller.
I don't use Bthrow alot for killing, and I find that it has a lot less lag then Fthrow. This allows me to get stage control and most of an SB. And I know about the repositioning, we talked about it in the metagame thread, so my point is just optimization.
Dthrow forces people to land on platforms, true comboing into Usmash. it is possible to DI, but I think it is DI up into jump at the higher percents.
At low percents, Dthrow leads into tech chase opportunities. They either, DJ away, Tech or fall. It is a completely advantageous situation regardless. That is what I meant by combo.
In regards to the second post, I am just trying to optimize Mewtwo as much as possible. His grab game is lacking, but I still find it worthwhile to look into. Plus, if I can get the extra 4-6% as well as be better set-up I will take it.
Looking at the staling conumdrum, I believe that it does 12% the first time. 11% the next 2-3 times and 9% after that, each tier it drops the KB goes down. However, you have to remember that there is a stale moves list. this means that only 9? moves can be staled at any time. So if you use Bthrow once, and 9 other moves twice, then Bthrow won't be affected. Shieks use this method to unstale their Fair to get kills easier.
So if I use Fthrow twice at lower percents, then I won't mind using Bthrow once at medium percents.