>1. In essence, the combine is the equivalent of adding many frames of open brood all at once, with continuing open brood being produced every day after that.
But the laying workers have had NO time to be suppressed and will kill the new queen long before that happens three weeks later IF they had open brood for three weeks which they will not because it will all be capped in 9 days.
>2. You only have to do it once. You don't have to come back every week to add another frame.
I've done it and sometimes it works. Sometimes you end up with a large laying worker hive.
>3. Whenever you are moving a frame of open brood, there is a chance you are moving the queen.
Not if you shake all the bees off.
> You would have to be careful about removing bees from the frames and then inspecting to be sure the queen is not onboard.
Correct, but shaking all the bees off is about 3 seconds work and checking is about 5 seconds work. If it takes you longer you needed the practice anyway. :)
>4. Unless there is a chance that the queen in the combined hive would be killed by the bees from the laying worker hive
A fairly good chance that she will or won't.
> it seems that a combine would be less work and more of a sure thing.
It is less of a sure thing.
>What's the downside to doing a combine?
http://www.bushfarms.com/beeslayingworkers.htm#other