trowel addict,more and more render systems dont use a solid bead , the system im using at the mo has only a solid base bead,on corners and window /doors we use a srim bead(think of a board bead but plastic ,glued to the inside of a ten inch wide piece of fibre mesh for the length of the bead).this beads job is mainly for impact protection and anti crack so you have to make you own angle hence the angle iron,then its used for the top coat(acrylic texture)to cover said angles again,keeps every thing nice and sharp.