It gave me some nasty graphics glitches and if you think about it, it's trying to do the same thing as FBlit so it's better to use one or the other and not both 
I'm not conviced that you're right yet

my memory tells me that you could save some chipram in os3.9 by doing this and that it caused no corruption for me.
But i will shut up now unless i has some real evidence. Until then you're right!

i would test it by myself ASAP if i still had OS3.9 installed on my AGA machine