What has accuracy got to do with it
Accuracy has everything to do with it.
The CFMU looks at the flight plans (demand), and the ATC capacity, and if the demand is too high over a certain period, then CTOTs will be issued.....all this I'm sure you know.
The CTOTs are issued as CFMU can assume that an a/c will be airborne between CTOT-5 to CTOT+10 (i.e. an airborne window of 15 minutes), and can then forecast what time that flight will hit the regulated sector.
If COBTs were used instead, CFMU would have to work on an airborne time of COBT+5 to COBT+60 (i.e. an airborne window of 55 minutes), certainly where I work. How would that be better?
I readily admit that the current CFMU system is a very, very blunt tool. I fail to see how going to OBTs would improve the situation.