Judging by the trials that are already in place, a single combined staff would be prohibitively difficult and time consuming to make for a lot of people. Combining existing trials would allow people to create as little or as much of a combination staff as they needed, and each trial's completion would see immediate results. For an item that is all about convenience, I think this sort of customisation and accessibility is important. Of course, in the ideal world a combined staff would be best, but realistically I think it'd require far too much work to actually solve any problems.