Great article! Being more powerful CPU means you can do same work with less energy spent, heat dissipation, which might justify bulk upgrades for a DC. Same time, means your business can grow with more resources standing by quickly.
Of course, simple assumption here, each set up is different but that's the basis for the upgrade.

Not only new process for CPU is cool for processing by itself, but means memory chips will follow similar improvements so you can add up cache, RAM and storage flash, which helps whole setup when refreshing capacity.

Haswell for servers is not urgently needed, considering better power saving features which help on laptop lines refreshed by mid-2013, but once it's ready for servers, they can benefit on a little more power efficiency which is always welcomed!