One thing that keeps bothering me about the FMAB story.(Spoilers for FMAB)

I was wondering, why wouldn't the homunculi just capture the Elric brothers and keep them in a cell until their plan was done?

There was always a chance that they would cause trouble for the big plan, so why risk it? The homunculi couldn't kill them, obviously, but why not just keep them locked up somewhere for both their safety, and the safety of the plan? Same for Roy. I know that the homunculi were really arrogant, but I'm not sure that's a good enough reason for them to take such an unnecessary risk.

The FMAB plot, in general, doesn't have too many plot holes that I can think of, but this particular detail keeps bothering me.