> Cutting your deployment time is equivalent to saving a one-off cost.
No, cutting deployment times produces ongoing savings in two respects:
1) Time is saved on deployment every release. That may be a small amount relative to the engineering that goes in ahead of releases, but it's definitely not a constant: it's proportional to the number of releases. When one considers not just major-version-every-six-month releases, but also maintenance releases, hotfixes, patches, etc., the time savings is increased even more.
2) As Joel discussed in the post, there are lost opportunity costs when a team develops a feature and does not immediately release it. Presumably, those features are intended to directly or indirectly increase sales/revenue: the sooner they are released, the sooner they can begin making the company money. This money is proportional to the number of features/improvements.
Investing the engineering effort required to achieve a shorter release cycle doesn't always make sense, but it definitely always produces value proportional to elapsed time.