You like climbing and you like to hit the descents. Sometimes you do lift runs, enduro races, sometimes you just want to session a hardcore line. The Extra II has been loaded up with features but still comes in a low weight package that gives the Extra more zip on the climbs and overall nimbleness.
Like all ARC8 bikes they are versatile - you could also call them balanced. The Extra is fitting for a lot of riders it will take an air or coil shock, has 160mm of travel and can be run as a mullet (with the help of the flipchip). It's simple, the flex stays require no maintenance and look great and the slider system has grease ports for purging and refreshing. And you fussy riders can quit your whining, there is head tube cable routing with ports to run moto style braking setups.
Nice neutral anti-squat values through the travel range and some progressivity built into the shock curve means your coil and air shocks will play nicely with the Extra. You really don't want loads of anti-squat as this will translate back into the pedals on rough terrain and reduce overall tuneability of the suspension curve - you an always use a climb switch on the rear shock but you cannot tune out excessive anti-squat.
The rear slider gives ARC8 the ability to closer tune the leverage curves, it offers loads more rigidity for the rear shock preventing side loading and increasing shock sensitivity. Bikes that implement the yoke style eylets are notoriously for wearing out air shocks and bending coil shocks due to the high loads transferred right into the shock body. This is not an issue with the new Extra, its as solid as it gets.