The organization where I now work has experienced substantial pain in the past by not upgrading SQL Server versions. For this reason, it has long been the policy here that we will continuously manage and move up the upgrade path.

I want to echo what several commenters have already said about the length of time it takes to upgrade a production environment. Even though we have a policy which states that we will constantly upgrade, we just succeeded in upgrading our main production instance two weeks ago, almost a year after 2012 was released. Further, we still have many production instances left to upgrade. Some of these will not be upgraded for at least another two years, due to the fact that they support 3rd party applications which themselves do not support 2012.

Given these examples, I believe that a sharp focus on licensing costs is a mistake. Delays in upgrading will lead to deteriorating systems. The longer the upgrade is put off, the more that it will cost in terms of time and effort to eventually upgrade. Systems in such a state are often simply decommissioned.