I've trained at a BJJ school that teaches throws.  When rolling, we would ask our partners if they would like to start standing (i.e., going for a takedown) or already on the ground.

Problem is that the throws are modified for BJJ.  For example, with the ippon seoi nage, as you turn away from your opponent, you go down to your knees and pull him down onto you to load him.  I imagine that the execution is probably a lot more likely to fail than a standing ippon seoi nage, since with standing, you can use your leg muscles to assist with the throw but you give that up when going down on your knees - which means that in the BJJ modified version, you probably have to be more swift in the execution.  Any hesitation, and your opponent is stuck on your back.

There's also the fact that going down to your knees when executing a throw, although perfectly fine on the mat, is a horrible idea in a real fight.

22 hours ago, Furinkazan said:

I've trained at a BJJ school that teaches throws.  When rolling, we would ask our partners if they would like to start standing (i.e., going for a takedown) or already on the ground.

Problem is that the throws are modified for BJJ.  For example, with the ippon seoi nage, as you turn away from your opponent, you go down to your knees and pull him down onto you to load him.  I imagine that the execution is probably a lot more likely to fail than a standing ippon seoi nage, since with standing, you can use your leg muscles to assist with the throw but you give that up when going down on your knees - which means that in the BJJ modified version, you probably have to be more swift in the execution.  Any hesitation, and your opponent is stuck on your back.

There's also the fact that going down to your knees when executing a throw, although perfectly fine on the mat, is a horrible idea in a real fight.

That just sounds like a drop seio nage (which is an established judo technique), unless i am misremembering! My club does similar and allows you to start either standing or on your knees. I think a large part of it is also the skill of the thrower: my judo isn't good enough (yet) so if i was to go for a turning throw like Seio Nage then i am more likely to give up my back to my opponent. If one of the multiple judo bb's we have at my bjj club does it you are likely in the air already before you realise the back was there!