The name FANUC RoboDrill is confusing to many with a 30 taper spindle. FANUC builds this model with a BigPlus spindle, also known as dual contact. The taper is ground but also the flange providing dual contact.
Another benefit beyond rigidity, is the finish achieved and longer tool life. Look at the drastic differences between BigPlus finishes and without BigPlus.
In the end, what really matters is how a machine cuts. The following are a couple examples that really show off the FANUC milling. If a dual contact spindle provides more rigidity, why wouldn't you consider this?