It doesn't look like the views custom breadcrumbs support in 6.x-2.0-beta2 really supports arguments.
I have two use cases for this:
My view path is path1/%, with the argument being a taxonomy term name. I want the breadcrumb to be My Menu Item | Term Name
More complex case:
View path is path2. If argument is present, it's path2/YYYYMM (calendar year and month), and it shows nodes from that year/month. If not, it's just path2, and it shows all nodes.
I want the breadcrumb to be My Menu Item 2 if there's no argument, and My Menu Item 2 | Month, Year if not.
I have used Views argument handling and it gets both of these partly right, but I need them to be correct, and I don't think there's a way to set this up in Custom Breadcrumb?