Lets see....
OEI and Flap Retraction above TOSA....
Gets you going faster with same thrust... a FAR bigger advantage since lift depends on square of your velocity compared to coeff of lift which is a function of your AoA. So you can climb quicker which is ALWAYS good because chances of rubbing against concrete or dirt is less in that particular configuration.
Another way to look at it is that IF THIS IS really that one day you are gonna get the OEI chances are you might get the other one going off too...
Something that sounded kinda like "
Gents in case you are in trouble hope for the best BUT assume and plan for the worst" said day in and day out in the flight school comes to mind...
Thus I would rather make sure I am wings clean after TOSA at Flap retraction speed instead of having extra drag in OEI situation.
If you think OEI and Flap retraction is high work load; I dare to presume both motors gone and having to retract flaps THEN will be beyond belief in terms of workload...
As for the statement:
"i think the main reason for keep retracting flaps in Boeing's standard procedure is for training purpose"
I sincerely hope you do not believe that... Stuff in those manuals and indoctrained in the Sims are things to make sure we have the highest probability of getting back in the air after an incident.
Safe flying buddy...