Artillery only divisions are the most efficient way to deploy artillery for several reasons. First, you get full leader bonus from the division leader and the stack leader. (Loose artillery only gets stack leader bonus). Second, you get fire power concentration because the artillery division will select an enemy division and only shoot at that division's elements. Finally, your artillery is effectively protected because, as long as you have no line elements in your arty division, the arty division will be targeted last. The combat engine is coded to attack support units only if there are no units with line elements (Cav or Inf) to attack. If you have a mixed division, there is a 25% chance that artillery shooting at it will shoot the artillery as counter-battery fire, but a pure arty division (like loose arty in a stack) will be targeted last.
When you combine all that with the fire bonus from supply wagons, artillery leaders, and 3+ level entrenchments on the defense, arty divisions are killers. (Esp. if you have 9+guns) So much so, I'm starting to drift to the side of the debate that considers it an exploit.
Artillery only divisions aren't something the developers had in mind when the engine was created. It really was a player innovation that came out of forum debates about the best way to use artillery (Shout out to the Grey Fox
). The engine classes artillery as support units and targets them last to avoid the guns been taken in assault phases too easily. The assumption that you would either keep them loose in a stack or stick them in a division with line units is pretty clear in reading the release manual. And of course more than half of the build pools for the CSA and USA have artillery attached to brigades that can't be un-attached (a subject of many forum gripes). Indeed, the practice is really rough on a CSA player, because the build pool is so limited for making pure infantry divisions. (And, of course, Athena does not build arty divisions.)
I would be less troubled by arty divisions if there was some chance that the unit would get targeted by line units, and you risked capture by assault or counter battery fire. A house rule of artillery only in mixed divisions or loose in stacks seems like the best fit with game balance as the division artillery fire is dispersed and risks counter battery, while the loose guns are safe from attack but limited by CP limits.
With that, I'll get off my soap box. In sum, artillery only divisions are the best way to organize your guns in a corps stack. The only reason not to is if you have a small division sized detached force (for the CP), or if you have a philosophical objection.