User blog comment:Danz The Pure/Item upgrade paths from weakest to greatest/@comment-1330314-20140319170100

The point of having super-expensive single-stat items is that you need to save up for a long time to buy them in one go. If you gave a build path, ADCs would just periodically leave lane to upgrade to a higher tier of efficiency without ever needing to take the additional risk of holding out or attempting to get a kill. The same would happen to mid lane: effectively, you're just making farming even more advantageous than it is already, because even the most expensive single-stat items can be bought over the course of a few trips. I think Season 4 already has a really long farming phase, and it doesn't need to extend it any further.