Idler arm J-29193 tool really necessary?

[rlyoung]

The manual calls for a J-29193 tool to set the idler arm taper before putting on the castle nut. I haven't taken the old arms off yet to know exactly what I will be working with, but this tool looks like a long threaded-rod coupler nut that you attach where the castle nut will eventually go. You torque it to a recommended amount, then remove it and put on the castle nut to it's torque setting.

Why the two stages? I don't quite see the reason behind this procedure if this is what it's all about. What special properties does this tool have over a shorter nut? Many people don't even use this tool apparently. They just put on the castle nut to spec. I don't want to cut corners, but I also don't want to spend money on something that one can really do without.

Thanks.